I was in a group where there was no evidence of shared ownership neither were there rules about how you disposed of a share. This led to massive arguments and bad-feeling (and threats of legal action). In fact, some of the group are still waiting for their money 3 years later.
If you do join a group make sure you keep copies of your cheque, photocopies of any and all documentation and, before you join, information as to how you leave the group.
Believe me, this is by far the most important element rarely considered when you join, but vital when you come to leave