Been in several ranging from 2 to 20 members. It's not the size of the group, but how it's managed that's important. Look carefully at the structure of the group (is there a clearly defined management system); the payment system, (is there an engine fund and does the balance match the hours left before the next overhaul/replacement); look at the booking system (is it equitable or does it favour a few members?) and the airfield (a strip with limited movements compared with an established airfield).
The best way to run a group without doubt is as a limited company. This way, the individual members get additional protection should something go badly wrong. One group I was in, one member was declared bankrupt and the bailiffs came to impound 'his' aircraft. But as he was a shareholder in the company with only one share with a nominal value of one pound, they went away disappointed. Also, look at the hours limits - many have a minimum of 100 P1 since award of a PPL and I would suggest this is sensible. Also, look at the way any check flights are done - is there a structure and is it held to? the same group with the bankrupt member was a bit strapped financially and needed more members. Being the only professional pilot, I did the 'check rides'. I objected to one prospective member , not because of his flying, but because he appeared to me to be a bit of a risk taker. I was overruled and he later proved me right by crashing the aircraft. And it helps to have a lawyer in the group!
Good luck.