I'm back.
I started a kindergarten co-op this year through my larger (~175 families) hs group. We already attend regular park days and various other activities, but my second ds was starting hs'ing officially, and I wanted something that was just for him (rather than tagging along with big bro), and that would meet regularly enough with a small enough group to make friends.
In the spring I just sent out an email with my ideas (i.e., weekly, structured but non-academic, etc.) and held a couple planning meetings -- amazingly everyone was on the same page in terms of goals and expectations. We met once informally over the summer, just to keep in touch, reassess who was in & out, and then started weekly meetings after labor day. We had six consectutive weeks, now a week off, then six more weeks. We'll likely continue a similar format after the winter holidays.
We have about 10 families who are committed, about 7-8 attend each week. We meet from 10-12 and do some free play to start with, then a story, craft, demonstration, game for about 30 minutes, then more free play and snack (everyone brings their own).
So far our topics have included names, comparisions (big, bigger), elections, fall leaves, gravity and bedtime. Each family chooses a topic and activities and hosts at their house, another members house, or a park. So far everyone has contributed equally and their seem to be no conflicts. I've heard no complaints, but there are a couple people who've dropped out (one seemingly b/c the time didn't work well, the other came once and never again, so I don't know what she thought).
Most of the families have been pleased with it, and the kids really have formed some friendships and look forward to our weekly meeting. The only issue we've had so far is that there is an abundance of boys, and only a couple girls (the parents don't mind, but I think the girls want more girls). Siblings are included and either participate, or hang out nearby doing something else.
When I first took this on it felt like it had the potential to become very....cumbersome. But planning has been super easy after the inital kickoff, and planning my week's activity was a little daunting (I was first) but so worth being able to sit back for the next 8 weeks and benefit from other parent's talent and planning. I am part of another, larger and more structured co-op, and I MUCH prefer the one I started, small, simple, informal.