I did the same thing for our class last year - at the request of the majority of the parents. Our school is a bit slow to embrace technology, and we (the parents) were looking for easier ways to connect with each other, chat about the school, share info pertaining to the school, and for some parents, send out birthday party invitations via electronic format.

Anyhoo... I called all the parents in our classroom, they offered up their email address if they wanted to participate (all but 1 did) and I then created a yahoo group, and sent them all invites. Really the invite thing is strictly to make it easier for them to join - AND - keep it a private group where others not in the class can't join. We got maybe 60-75% participation on the yahoo group. Most of the chatter was centered around the schools opera (volunteers needed, etc.), end of year gifts, new baby gifts (teacher was expecting last year) and summer planning. Overall not a very active group, but still has served its purpose.
As far as your group - it sounds like it was started for similar reasons. Often I've found the parents want ta place to think aloud with other parents about various policies, upcoming events, etc. That is usually the reason to exclude school personnel. As for safety, be aware that anything in writing (email or otherwise) can be located no matter how private you think it may be. I'm pretty candid in person, so while I may say something sooner in email on a private list than I would in person, I'm not fussed if what I did at one point say eventually comes to light, kwim? However, if you are an off the cuff ranter, you may want to temper your messages until you get a comfort level established.
