I've never been in a group but looked at a few shares for sale and checked out the background.
Obviously much depends on intended usage.
To finish one's PPL or to do additional Ratings, one needs a G-reg on a Transport CofA. A G-reg on a Private CofA can be used if you or your spouse own it outright, or if the instructor does not charge for the flight training. An N-reg is a whole can of worms.
If one wants to fly IFR, one needs a group whose members are rather more solvent than average, and the group has a history of getting things (avionics) fixed promptly, without arguments. This factor breaks up a lot of groups; many for-sale shares are from members who are fed up with lack of agreement on maintenance standards, and IFR capability is a common one. As well as obviously needing a reasonable aircraft.
If one wants good access, it's no good getting into a group with say 25 others - unless you are happy to fly weekdays only. I know somebody in a 25-group who does this and has very good access.
You need to get on with the others, so the agreement is (hopefully) not invoked.
But it seems to me that it pays to get it right at the outset, because selling a duff share can take years.