What Legal Documents Does a Startup Need Before Adding a Co-Founder?

Before adding a co-founder, a US startup should document the role, decision rights, equity economics, vesting, intellectual-property ownership, confidentiality, and what happens if either founder leaves. If the company will issue stock, it should also approve the issuance correctly, update its capitalization records, address securities-law compliance, and give the recipient prompt tax guidance, including the 30-day deadline for an 83(b) election when it applies. The exact package depends on the entity, state, tax treatment, and facts, so use this as a readiness checklist and have qualified startup counsel and a tax adviser adapt it.

Educational information only. This is not legal, tax, accounting, or investment advice.

Start with the relationship before the equity percentage

“How much equity?” is important, but it is not the first question. Before negotiating a percentage, the founders should write down the working relationship they are actually creating:

each founder's role, expected time commitment, and start date;
responsibilities and measurable near-term outcomes;
salary, expense, and outside-work expectations;
who decides product, hiring, fundraising, spending, and strategic questions;
which decisions require both founders or board approval;
what each person has already contributed;
what happens if a founder stops contributing, leaves, dies, becomes disabled, or is removed;
how disputes and deadlocks will be handled. This document may be called a founder agreement, stock purchase agreement, restricted stock agreement, or a set of board and employment documents. The name matters less than whether the full package tells one coherent story. A vague handshake such as “we are 50/50” does not answer who owns the code, whether either founder earns the shares over time, or who can bind the company. Those unanswered questions become more expensive after the product, team, or valuation grows.

Confirm the entity and its governing documents

If the business has not formed an entity, decide whether the new person is joining a project, a partnership, an LLC, or a corporation before promising “shares.” Each structure creates different ownership, tax, governance, and filing consequences. For a corporation, review at least:

the certificate or articles of incorporation;
bylaws;
current board and officer records;
stock ledger and capitalization table;
existing stockholder, voting, investor, or right-of-first-refusal agreements;
any equity incentive plan;
prior SAFEs, notes, warrants, options, or side letters. The goal is to confirm that the company has enough authorized equity, that the promised security exists, and that the proposed rights do not conflict with earlier agreements. Delaware corporations should pay particular attention to formal approval. Section 141 of the Delaware General Corporation Law places management of the corporation's business and affairs under the board's direction. Section 152 provides that the board determines the form and manner of consideration for stock and may authorize shares to be issued through a board resolution. Other states and entity types have their own rules.

Document the equity grant and vesting

The equity documents should specify:

security type and number of shares, units, or options;
purchase or exercise price;
percentage only on a clearly defined capitalization basis;
vesting start date, schedule, and cliff;
what service earns vesting;
the company's repurchase right for unvested shares;
treatment on termination, acquisition, death, or disability;
transfer restrictions and any right of first refusal;
information, voting, board, or observer rights;
representations by the company and recipient. Use share numbers and a current cap table, not just a percentage in a message. Percentages change when the company issues more equity or raises money. The SEC's startup securities guide notes that common stock is more commonly issued to founders and that ownership is often reflected as a percentage or number of shares on the capitalization table. Vesting is not a signal that the founders distrust one another. It is a mechanism for matching ownership to continued contribution. Without it, a founder who leaves early may retain the same stake as the founder who spends years building the company.

Approve the issuance and update the records

The company should create a clean approval trail at the time of the grant, not months later during financing diligence. Depending on the entity and transaction, that may include:

board consent approving the recipient, security, amount, price, vesting, and agreements;
stockholder approval if required by the charter, governing documents, law, or an existing investor agreement;
a signed stock purchase, restricted stock, option, or unit agreement;
payment evidence when the founder purchases the security;
an updated stock ledger and capitalization table;
required securities notices or state filings;
delivery of the final signed package to the company and founder. Stock and options are securities. A private company still needs an available registration exemption and must consider applicable state requirements. The SEC explains that Rule 701 can exempt certain compensatory sales to employees, consultants, and advisers by non-reporting companies, subject to conditions, limits, and disclosure requirements. Founders and their counsel should confirm the right exemption for the actual transaction rather than assuming every founder issuance falls under one rule.

Assign pre-existing and future IP to the company

The company should identify what each founder created before joining and what they will create for the startup. The documents should address:

source code, models, product designs, inventions, domains, brands, datasets, content, and documentation;
pre-existing IP that stays with the founder;
any license the company needs to use excluded IP;
assignment of startup-related inventions and work product;
disclosure of open-source and third-party components;
cooperation with patent, trademark, and copyright filings;
return or deletion of confidential material when the relationship ends. Do not rely on “work made for hire” alone for every contribution. The US Copyright Office explains that employee work within the scope of employment can qualify as work made for hire, while specially commissioned work qualifies only in specified categories and with an express signed writing. A separate present assignment of relevant intellectual property is often used to address work that may not fit that definition. Patent ownership needs its own attention. The USPTO explains that ownership can be transferred through an assignment and that the supporting assignment documentation can be recorded through Assignment Center. If a founder filed an application personally or invented before the entity existed, identify and document the chain of title instead of assuming the company owns it.

Put confidentiality and outside obligations in writing

Each founder should disclose existing employment, consulting, university, grant, invention, non-solicitation, or confidentiality obligations that could affect the startup. Questions to resolve include:

Did a former or current employer fund or direct the work?
Was university equipment, grant funding, or lab time used?
Does another agreement claim inventions related to the founder's field?
Is the founder permitted to work on the startup?
Is any code, data, customer information, or model subject to third-party restrictions? The founder confidentiality and invention-assignment agreement should protect the company's information without asking the founder to misuse someone else's secrets. The clean outcome is a written record of permitted contributions and excluded materials.

Address the 83(b) decision immediately

When a founder receives substantially nonvested stock, an 83(b) election may be relevant. The decision is tax-sensitive and depends on the transaction. The IRS's 2025 Publication 525 explains that restricted property is generally included in income when it becomes substantially vested unless the recipient makes a valid choice to include the value at transfer. The IRS also provides Form 15620 for a section 83(b) election. The deadline is short: the IRS instructions say an 83(b) election must be filed no later than 30 days after the property is transferred. It is not a 30-business-day period and the election is generally not revocable without IRS consent. The company should not make the founder's personal tax decision, but it should deliver the signed equity documents promptly and tell the founder to seek tax advice immediately. An 83(b) election is not available for every equity instrument. For example, Publication 525 says the election cannot be made for a statutory or nonstatutory stock option. Confirm the security and tax treatment before using a form.

Clarify control, deadlock, and departure

Two founders can unintentionally create permanent deadlock. The governing documents should identify:

board composition and appointment rights;
officer roles and authority;
actions requiring board or stockholder approval;
reserved matters requiring a founder's consent, if any;
bank, contract, hiring, and spending authority;
dispute escalation;
removal and resignation mechanics;
repurchase, transfer, and right-of-first-refusal provisions;
treatment of vested and unvested equity after departure. Avoid giving a title, board seat, veto, or signing authority only because it feels symmetrical. Give each right deliberately, record it in the appropriate document, and make sure the company's bank, cap table, contract process, and internal systems match.

A practical co-founder document checklist

Before the new co-founder starts or receives equity, confirm that:

the role, time commitment, compensation, and decision rights are written down;
the entity and existing governing documents have been reviewed;
the security, share count, price, vesting, and departure treatment are clear;
the company has authority and enough authorized equity for the issuance;
required board and stockholder approvals are signed;
the purchase, restricted stock, option, or unit agreement is complete;
the stock ledger and capitalization table will be updated;
the federal and state securities-law basis has been checked;
pre-existing and future IP are identified and assigned or licensed correctly;
confidentiality and outside obligations are addressed;
the founder receives immediate tax advice about any 83(b) decision;
board, officer, signing, deadlock, and exit rules are documented;
final signed copies are stored in a diligence-ready location. The objective is not to predict every disagreement. It is to make ownership, contribution, control, and exit expectations clear while the founders are aligned.

Frequently asked questions

Is a founder agreement enough by itself?

Usually not. The relationship may require corporate approvals, an equity purchase or grant agreement, IP and confidentiality documents, cap-table and stock-ledger updates, tax notices, and changes to board or officer records. Treat the founder agreement as part of one coordinated package.

Should co-founders always split equity 50/50?

No universal split is correct. Consider prior work, future commitment, role, risk, cash invested, compensation, and the vesting structure. Equal economics also do not require identical operating authority or permanent board deadlock.

Should founder stock vest?

Vesting is common because it ties ownership to continued contribution and gives the company a repurchase right over unvested shares if a founder leaves. The schedule, cliff, acceleration, and departure rules should reflect the actual relationship and be documented at issuance.

Can the company promise equity before it is formed?

People can agree on intended economics, but the future entity cannot issue stock before it exists. Formation, authorization, approval, grant documents, and cap-table records still need to be completed. Counsel should also review whether the pre-formation conduct created a partnership or other obligations.

Who files the 83(b) election?

The service provider receiving the restricted property makes and files the election. The IRS instructions require filing within 30 days after transfer and delivery of required copies. The founder should confirm applicability and filing mechanics with a qualified tax adviser.