Not a silly question at all. The reality is that each group agreement varies to suit the needs of the group. It's why, when looking at a share, you should always look very carefully at the group agreement in some detail.
Some groups do allocate specific dates for each member so there is a bit of a lottery about whether the weather will be suitable on your allocated date. However, most groups seem to limit the number of bookings a member can make in advance (ours allows 3 bookings in the diary at any time) so one member cannot hog the aircraft.
There is no real standard agreement as regards finance. Ours requires members to pay a monthly (to cover the fixed costs of CAA fees, hangarage, insurance and part of the annual maintenance) and a wet rate per hour to cover fuel and something in the pot for engine replacement. We also agre that in case of a major and unexpected expense, then we have to put our hands in our pockets.
Wet rate means that fuel and oil costs are included in the hourly rate, dry rate means you have to pay for your fuel on top. You still need to pay for fuel but this is offset against your flying bill.
The benefit of the wet rate is that you know how much an hour's flight will cost. The downside is that, as fuel prices vary, the cost to the group might suffer if a member always tops off at an expensive airfield, so some put a limit on the reimbursement.
Some groups insist on a high monthly (possibly incorporating 1 hour of flight) so as to build up a healthy bank balance - always a good thing.
Our agreement states that landing fees away from base are the responsibility of the pilot and not reimbursable - simple really. If we had to pay for our landings at our base, then this would be added to the cost of the flight for the pilot. Fortunately, for us, we pay no landing fees at the moment.
Group agreements should also include details of your responsibilities as a member, and must show how you can leave the group. Some don't, so don't go there!
In short, you should not just look at the advert for a group share. Ask around to see if anyone knows about the group, find out why a share has become available, and, if still interested, meet them and discuss the details.
Buying a share (or joining a non-equity group) is like a marriage. We all go in with high hopes, and a proportion of us will end up in an acrimonious divorce. It is better to look at it in strictly business-like terms as aviation is not a cheap activity, and most group members have limited funds.
Getting it wrong can be very expensive.