Short answer: Yes, but you can co-GP as long as you follow some defensible guidelines:
To defend a charge of acting as an unlicensed broker, you have to be able to prove you had an active role in management other than raising money, and that’s what you got compensated for. What’s illegal is “transaction-based compensation” based on the amount of capital you raise. The more GPs there are in a deal, the harder this is to defend.
Further, you must be admitted to (or employed by) the company that is selling the securities, as the securities exemption being claimed — usually Regulation D, Rule 506 — only applies to the “issuer” of the securities.
Bottom line: Make sure you have an active, documented role in management, and you don’t get compensated using any formula that plugs in the amount of money that you raised.