What a good question.
My experience is that the utilisation is focused around short hops (much as yours) with little, if any, touring. However, very often pilots will want to enter competitions. The "problem" with that is there is high demand for the aircraft over competition weekends (often the time everyone wants to fly anyway) and of couse competitions involve taking the aircraft away for the day / weekend. Some thought therefore needs to be given to the profile of the group and what each member expects. This can mean that whilst at first blush the group can be structured around a large number of members with everyone being satisfied with availability in reality it is not quite that simple. Even tow members going to the same competition causes more problems than you might imagine, but with organisation can work.
In so far as cost is concerned I have never operated an Extra, only enjoyed flying them (a bit). I would guess that as ever the hire rates are not a bad guide as to running costs but also, as ever, so much depends on how the capital cost is financed. However, putting together your own budget should be relatively straight forward. I would be amazed if the group would do more than 150 hours a year (very few do), so factor the costs around the required maintenace, hangarage, insurance, fuel and unexpecteds and you will not go too far wrong. Ultimately, unless you are very brave, the members are going to have to meet any shortfall anyway.
You probably are already aware this is a very specialist market. It is tuff both getting people to join groups of this type and even tougher to find a market for the aircraft should the group decide they want to sell.