our playgroup is just 5 moms who were in childbirth class together - our original set is 3 (almost 4) now. we used to meet at each other's houses, but now that there are 10 kids, we meet at a public place (park, indoor play area, etc). we don't have any rules per se, but we no longer have playgroup at each other's houses b/c some of us notice that one or two of the other moms have/take no control over their kids. very frustrating. they just chat and chat and don't notice/care/pay attention to what the kids are doing.
that said, i defintely go to playgroup/have play dates b/c I like to interact w/other moms, BUT not at the expense of our kids. if the kids (theirs or mine) want to read or play w/me, I comply OR sometimes will play/read for a few minutes and then explain that i want time to talk w/the other mommies. If i notice (and I'm usually the only one that does) that one is being hurtful, i usually intervene and/or bring it to the offender's parent's attention if necessary. (very annoying to have to do so imho!)
same thing goes for when my kids are at a public play area. i stand back whenever possible and allow my kids to play on their own, i don't intervene unless necessary or requested to do so by my kids, i let them play. (I do like a break now and then!) But it chaps my ass when parents aren't paying attention and their kids are running amuck and/or hurting other kids (there's always at least one - trust me!) I mean it is NOT a 2 yo's responsibility to act like a well-mannered human being. it is his/her parent's responsibility to be paying attention and intervene when necessary.
that said, i do think that a 5 yo should be aware of littler ones and if not, should have a parent making them aware - same for any age child. if my 2 yo bumps into someone accidentally OR hits someone on purpose, i know about it and help her to act appropriately and/or fix the situation.
WOHM married to SAHD, living the dream w/our: 3 girls (14,12,10) and 3 boys (7,5,3) and tie-breaker due Jan 2014