Apart from flying the aircraft, there are many other issues to consider such as the group structure/rules/constitution, availaibility and the maintenance, condition and equipment of the aircraft. Does the group have any reserves, and what happens with any unexpected bills? And do you like the other group members? Will you get on with them? Are there any hours builders? Try to meet, or at least speak to on the phone, more than one other member.
I had a group trial a few months ago - I offered to pay at least the legal half of the direct costs but he was having none of it - presumably because he knew I was genuine. Unfortunately I didn't buy in!
There was a comprehensive article in Pilot a few months back about group shares.