In my experience, support groups generally have similar practices. Things like:
* There is a facilitator who is responsible for enforcing the rules/guidelines and making sure that everyone has an opportunity to participate. Ideally, they also keep an eye on how people are doing and step in if it looks like someone is having a hard time with what is going on.
* People sit in a circle and only one person should be speaking at a time. You're not supposed to have cross- or side conversations.
* The group generally begins and ends with going around the circle and people saying their name, and sometimes their preferrred pronouns and/or some short comment or response to a question. (E.g., what's your favorite ice cream?)
* People are supposed to use "I" statements and not give advice, judge, or question someone who has spoken.
* What people discuss and reveal in the group is confidential.
* Although not usually made explicit, no one is forced to speak who doesn't want to. (You might have to listen, though

)
In practice, groups vary a lot. The mix of people and the skill and attitude of the organizer(s) and facilitators make a huge difference. But this is what is expected.
In my experience, you get people at all points in their transition or non-transition or figuring-out, and a halfway decent group should allow space for all. It is a good idea to have some idea of what kinds of trans people there are in the world and what the typical concerns and triggers are, but if you've been lurking at Susans for a while, you've probably seen a lot of it.
Being quiet and listening a lot is always a good policy for the first few sessions of any kind of group, and it's true here, too.
My own interest in support groups was originally so I could see what real trans people (esp. trans women) were like, so I could figure out if that was a direction I would be comfortable going in. Since then, I've been going to find people and groups where I could feel safe talking about all of the confusing and distressing things I'm going through inside.