Yes (answering OP) - I've been involved in parent forums in 3 different schools (well, 4 if you count the one when I was a teacher rep - the other 3 were 3 schools my dc attended).
All quite different.
All valuable.
There is no point though, unless you have a senior manager there who can
a) take ideas from parents back for discussion and
b) explain to parents why the school does something this way, or isn't able to do something that way.
Some have 'reps' - perhaps one per class - I presume that must be where there are a lot of keen parents. All the ones I've been on, every parent is entitled to attend, but it ends up with half a dozen, unless there has just been a controversial change or OFSTED report when they swell to about 45 people turning up.
The Primary school one used to have one each half term, on a rotating basis of 9am (once dc had gone in to school but you were up there anyway), then the next meeting was at 2pm (so finishing as the dc came out), then the next one was at 6pm so folk who work outside the home can come.
The (better) secondary one also did a lot by e-mail - only people who had shown some interest, but it was helpful when you could only make occasional meetings.
I think all were useful to some degree.