There are some professional shared ownership schemes for boats, but none that I have come across for RIBs. This would mean setting up your own scheme, and it will be vital to make sure that you a proper agreement drawn up.
Here is some food for thought from someone considering a similar idea, for a canal boat. The detail will be different, but the concepts are sound.
There are interesting threads in uk.rec.waterways here
When you are looking for potential partners, you could try uk.rec.boats.power and also joining BIBOA
and putting a nitice in RIBlines, the club mag.
Hope this helps
[This message has been edited by John Kennett (edited 30 July 2001).]