<-------- Patent lawyer
DISCLAIMER: I'm not your lawyer and my comments in this post/thread are not legal advice. You rely on anything I say in this post, thread or forum at your own risk.
Your question is complicated and the appropriate course of action is often fact specific. In the scenario specified, one course of action might be to form the business, and have the owners of the IP assign (or sell) the IP to the business for an ownership interest. The amount of interest in the business each of the IP owners take in the business may depend on any number of things such as the nature of the IP (copyright, trade secret, patent, trademark, etc.) how ownership of the IP is divided, the degree to which each owner contributed to the IP in question (e.g., in the case of owner inventors), etc.
The business could also be formed and take a license to the IP in question . . . but that is a more complicated scenario. . . .
In any event, you should contact a competent business attorney who understands IP, an IP attorney who understands business formation, or both.
Good luck,
Sox