There was a brief discussion about this previously at http://www.rib.net/ubb/Forum2/HTML/000030.html
which has some links to look at for ideas.
Formal shared ownership is relatively common on canals and rivers, but I don't know of any schemes for RIBs. There are less formal groupings which may or may not work so well. The 'Yellow Tang' team seem to still be friends, despite the trials of having to replace their engine recently -- I'm sure Alan will be able to add some useful thoughts!
One drawback (in the UK at least) is the unreliability of the weather. Most shared ownership schemes allocate weeks to each member well in advance, which doesn't allow for spur of the moment "the weather looks good, let's take the boat out" activity.
Motor Boats Monthly ran features about private syndicates in the May 2001 issue.