I'm assuming the amounts of money involved are likely to be fairly small. If other musicians feel they're being short-changed or excluded, it's more likely that their feelings have been hurt rather than the money.

My limited understanding of copyright, as it applies in the UK, is that a performer owns the copyright in their performance. You cannot therefore use their performance without their specific consent. Legalities aside, if these musicians are still your friends, then you shouldn't want to do anything to upset them. If they're no longer friends, the same applies - things could get really messy if they take umbrage!

Of course, if the amounts of money are significant, then you definitely need legal advice and proper agreements drawn up.

When my band recently made a CD we all contributed equally to the cost and we are sharing equally in the proceeds. I did the bulk of the project management and admin, mainly because I had time on my hands, but didn't take anything extra for it. neither did other band members for the non-musicial assistance they gave - we regarded it as a project for the whole band. However it is important to sort these things out at the start, no matter how reluctant the others may be to discuss it.