I've been to two different ones in my state and I don't think there's any one way they run the meetings. I think the general guidelines are that they are meant to be semi-anonymous and confidential, similar to like AA or Al-Anon. Although there isn't necessarily a focus only on the LGB, I think it's the exception to find a group that has a sizable trans presence...which may not really matter to you if you intend to go stealth.
The first one I went to a few years ago was pretty terrible, honestly. I went with my mom and identified myself during the introductions as trans, which then turned the group into a "let's gawk at, interrogate, and misgender the trans person" session. I never went back. Since that won't matter to you, I would estimate that about 75% of the group was made up of SOFFAs, 25% were LGB and mostly under 21. There were about 20 people there. The group stayed in one discussion group the whole time and apparently they alternated each month on having a speaker or having an open discussion.
The group I currently attend is a much larger and much more diverse group. It regularly sees 50-60+ members, of which about half are LGBT people and half are SOFFAs and there is a significant number of trans people and family/allies that regularly show up. After an introduction and a speaker (if there is one), we break into 3-4 focused discussion groups and talk about whatever. Sometimes there is a separate youth discussion group if there are enough of them and if they have a facilitator volunteer to keep the kids on track.
It probably won't hurt to attend a meeting to see if you like it, if it isn't what you're looking for you don't need to go back.