I have a question about the "founder" status. I've been working on my project for quite some time and it's taking off. It's very likely I'm going to be able to afford inviting a friend on a part-salary/part-equity basis in the next few weeks. I'm still going to have a vast majority of shares. Will it be OK to count him as a co-founder?
Seen as the people at YC HQ will be in bed for another few hours I might as well offer my opinion. Your description appears to me to be more like an employee relationship than a co-founder relationship.
A co-founder is generally someone who's goals and interests are closely aligned with yours. Someone who's in it for the long run through all of the peaks and troughs experienced on the startup journey. If you own the majority of the shares and your friend's main interest is the salary you're paying him/her, then his/her goals are not closely aligned with yours.
I don't know how YC will view this but my advice is to think about building a team of co-founders who will stick together through thick and thin and share in the success of the company as well as the struggles. I guess that's why its so important to get good co-founders. You don't want to give half your company to someone who you don't believe offers a whole lot to the company's success. I hope this helps.
To simply answer your question, I believe the general rule is 15% or more and they can attend YC.
If you are simply adding him as a cofounder so that you are not listed as a single founder, don't do that. It may seem like you have to have a cofounder to get into YC, but founders who are new to working together are a bigger red flag than a single founder, and there were several single founder companies in the last batch.
