When, Where and How Often
Is the group to meet for a specified number of occasions? Or is the implication that it will meet for ever? Perhaps agreeing to meet for a limited number of times and then reviewing things together is the best way.
Can anybody join? That may seem like a good idea but the danger is that you will get a group of people whose needs are very different and perhaps incompatible. The alternative is to invite people to join but the danger of that is that you may seem elitist and exclusive to those not invited!
Is there an optimum size for the group? Eight to ten people seems a good number but remember that not everybody will be present every time.
How long will the meetings last? It's a good idea to establish agreement about this and then to end at the appointed time. Some members may have other commitments to keep.
Where to meet? A public place, like a church room, may be easier for people to come to but may well be less comfortable than somebody's front room. Should you always meet in the same place, or should you move around to different venues? There are advantages and disadvantages either way.