IO is right and it is people who matter. I have been a treasurer and a secretary in two different groups. In the first, being treasurer was hell as one person in particular would not pay and the group for some reason would not agree DDs or payments in advance. You need a few active people to take on the principal roles: usually as a minimum a good treasurer and someone who can liaise as required on technical issues. I was a member of Lister's group for a while and two such people (and others) made quite a large group run very well. Generally though small is probably better.
You have to make your own judgment. Ask questions, meet as many of the group as possible and don't let heart rule your head because you like the aircraft.
As to the present problem, face to face talking to the treasurer is the way forward. He certainly should not be using his own account for the group's finances. Beyond that, it is difficult to make any constructive comment, not knowing the individuals concerned. Trying to get a few newer, fresher faces in will help, even if that means expanding the group.