Hi Kef
I think that FFF already answered most of your first question.
There are as many ways of sorting out booking systems and rules as there are groups. Make sure that the group you join has rules/booking system that cater for your kind of flying/ideas.
As FFF stated the ability to join a group depends very much on the type and the groups attitude. There may be insurance requirements too but they can usually be sorted out without too much heartache.
Most groups are formated around equal shares so that all members have equal rights to the machine. Some have rosters and some operate on a first come basis.
The PFA website has a standard contract which is a good starting point. Appreciate what FFF says but even when we can trust each other it is worthwhile to ensure you have a contract and some house rules. Things that can get sticky are best sorted and agreed when everyone is still very much in luv.
It is said that most well run groups never have to advertise and some even have 'waiting' lists for prospective members.
Best thing (IMHO) is to go and have a look around at the airfield(s) that are the ones you want to fly from, and have a chat with the refueller, guy/gal in the tower, airfield manager, maintenance people. They are usually in the know about what is happening and where the vacancies are or may be.
It is certainly the way we got further group members.
There are plenty of flying schools and clubs that manage private aircraft with a PT C of A for owners. It saves them the capital outlay and usually provides them with something a bit more exciting than the bog standard flying school material so that they can cater for a wider public.
Again I think it pays to do some investigations beforehand and ensure that all is crystal clear and well formulated in a good contract. Have no idea but would have thought that AOPA may be helpful if you need further legal advice re this option.
Have fun looking around and when you are serious have a hop in your prospective mount. Ensure to offer appropriate compensation for the pleasure, many a time it will be waived, but it enables the owner/syndicate to see that you are serious and not a tyre-kicker
FD