How you divide profits with the attorney would have a lot to do with what the attorney is doing on an ongoing basis.
Is the cost of producing the PPM docs part of his contribution or is that taken from the general funds? Will the attorney handle the accounting and reporting to and managing the investors? Will any legal maters that come before the group be handled by him as part of his job as a manager or would it be billed separately? These types of questions can help decide what is a fair split long run.