Most founding teams are not born complete. A technical cofounder joins eight months in, a commercial one after the first pilot. Bringing someone into the founding circle late is normal. It is also the moment I saw go wrong more often than almost any other, for one reason: the newcomer's arrival changes everyone's deal, and teams treat it as a handshake instead of an amendment.
Start with the number, since that is what everyone thinks about first. A late cofounder rarely gets an equal share, and should not by default. The early founders carried months of risk, unpaid work, and the uncertainty of the blank page. The honest way to set the newcomer's slice is to price the future, not the past: what matters is the four years ahead, the role, the scarcity of the skills, how far the project would move without them. A useful test I give teams: would you rather have 100 percent of the project without this person, or your diluted share with them? If the answer is not an easy "with them", the conversation is about the person, not the percentage.
Then the mechanics, which protect everyone including the newcomer. Full vesting on the new cofounder's equity, standard schedule, real cliff: they are the newest bet at the table, and vesting is how a bet stays fair. IP assignment from day one of their involvement, not from the day paperwork catches up, because the code and the designs they produce in the enthusiastic first weeks are exactly what a future dispute would fight over. And clarity on titles and decision rights: "cofounder" is a word that means whatever your agreement says it means, and nothing more.
Now the part teams skip. Your existing founder agreement was written by and for the original team. The day a new cofounder joins, that document is wrong: the equity table it records, the decision rules it sets, possibly the unanimity it assumes. You do not fix that with a welcome dinner. You fix it with an amendment that all of you sign, newcomer included, so that one document describes the actual team. I litigated the alternative: an old agreement signed by two, a third person who "was obviously a cofounder" in everyone's memory and nowhere in writing, and three lawyers reconstructing intent from Slack messages.
A living agreement makes this a ceremony instead of a crisis: simulate the new split, see what everyone's stake becomes, amend, sign, done. The newcomer starts their first week inside the deal instead of orbiting it.
Bring on the person your project needs. Just make sure that on the day they join, the paper and the team describe the same company.