We were six couples originally, and added one couple. No one has left the group, although like anything, some play more than others.
On weekends we usually get together in two groups, although people don't stick to just one group. It depends on who is hosting, when, the location, who else will be in a particular group, etc. The emails and texts are flying during the week making arrangements. It's also not unusual for spouses to split up between groups for a particular get together, depending on where a favorite play partner will be. Around every six weeks we all get together.
During the week there is a lot of alone play, threesomes, and swaps where the spouses switch watching the kids while the other mommy and daddy are elsewhere fucking. Explaining to the kids may be a problem when they're older.
The one couple that joined us after we initially formed weren't even really swingers, they had just done a little honest playing on the side. They knew about our group, discussed it and really wanted to join. We all talked about it, and tentatively let them start the process. First, the pledge to be monogamous between themselves, starting the multiple STD testing, and in addition come to some of our get togethers for soft play, let us watch them fuck between themselves, and they watch us. Although they both had had partners outside their marriage before, nothing like our group arrangement. We wanted to be sure that they both had no problem with a group environment, casual swapping, and him and her being hands on with multiple people. Everything went fine (the women talked with her, the guys with him during the week to assess where their heads were), they passed the STD tests, and we voted unanimously to let them join. They are also very fit and attractive. We had an everybody attends party after they were accepted. I worried that maybe it was too much for the first time, especially for him seeing his wife getting a lot of attention, but all is fine. I wish we could find another couple like them.