I've only been in one group so far. We decided to set the hourly rate as: estimated cost of fuel and oil, engine fund, plus a bit extra for contingencies. The monthly charge covers insurance, hangarage, maintenance, companies house (£15 a year), accounts (done for nominal charge or free). People pay their own landing fees (free at the base airfield), club subscriptions etc.
As for the engine fund, with a Lycoming AEIO-360-B2F (injected, inverted fuel and oil) we budgeted for £15,000, fitted, at 1500 hours, and calculated the hourly rate accordingly.