The piano group that I started just goes from house to house, with members hosting. We play on a variety of pianos, which is a good thing. There are 7 of us that regularly host, and our attendance is usually around a dozen. No rules, no dues.
We have never advertised or put up flyers or used the interwebs to promote our group - just word of mouth. Since we do invite people into our homes we tend to be a bit more cautious about new members. The current members spread the word and we pick up new ones and some old ones drop out. We have been meeting regularly for 5 years now.
My other piano group is much more formal, and regularly meets at PianoWorks in Duluth, Georgia. So it has rules and dues. It also has a much higher membership, but the members tend to be piano teachers or people that have been playing their entire lives.
So the two groups are quite different, but both are great.