Thankfully it is a while since I was in a group - so you'll guess my view is a little jaundiced!
In the first group I was in we had 20 members and monthly payments that covered all the standing costs. The hourly rates covered fuel, engine hours and an element of the maintenance. An arrangment that you would think is pretty fair. At that time it meant the monthly costs were between 1 and 2 hours worth of flying. I did around 30 hours a year and was about the 3rd highest user. The amount of complaining over the rates had to be experienced to be believed! The largely non-flyers, believed their monthly rates should be cut because the aircraft was flying! The users (only about 7 of the 20) thought the costs were fine but wanted a better radio etc - which of course the non-users did not want to pay for.
Thankfully we had a strong chairman and through his efforts it all stayed together. But the sniping and general resentment had to be experienced to be believed.
Latter I shared an aircraft with 2 others. Here we simply split all the costs 3 ways (except aircraft upgrades where the owner asked for contributions and fuel and landings). It worked pretty well - but we were (and still are) friends and probably it could not work with a larger group. It relied on us doing the 50 hrs checks and an element of the annual work under supervision - again something that whilst it works with a small group probably would not with a larger one - where of course only the core people actually do the work.
On reflection the model where monthly costs cover the majority of expediture are probably the way to go - along with a robust set of rules. Hopefully you will never need the rules but unfortunately there always seems to be someone trying to take advantage...