Probably about 4-6 families each session, although some venues are more well-attended than others. The one I mainly go to is in a community centre in a park. The park is a really good one and there's a lovely child-friendly cafe where we often move on to after the group finishes.
All the groups are drop-ins. With a child on the spectrum it's very stressful to 'have' to be some where by a certain time, so knowing that I can drop in if it suits on the day is very nice! So many events that are organised for us are pre-booked and ask for notice of cancellation, which I tend to avoid as I feel guilty if a meltdown means we can't go!
It's mainly mums who come, although some families come as a family, which is lovely as the Dads get to play with their Dcs while the mum gets to chat with other mums. I usually take my 3DSs on my own (while DH does the hoovering 
), but they are always happy to go off and play so I can have a natter (doesn't happen anywhere else!)
Yes, Leisure services staff the trampoline. I don't know how insurance is organised, but I assume you need some insurance for the group anyway, so couldn't you just check it covers for 'active' activities as well as general arts and crafts type things?
I prefer events that are inclusive, as being excluded for having the 'wrong' type of disability is just as bad as being excluded for having any disbility, IMO!
I think one of the problems with our group is that it's hard to get across what it is...Playgroup sounds like it's for pre-schoolers, coffee morning sounds like it's for pensioners...it's not a 'support group' ("My name's amistill and I am the mum of a child on the spectrum" - Very Alcoholic's Anonymous!). I tell all the parents I come into contact with about it but many feel they won't fit in, which is a shame. It's really a bit like a face to face Mumsnet: we all sit around ignoring our DCs while chatting about inane topics or setting the world to rights!