The style of a therapy group depends on who is running it. Before joining a group, you should talk to the therapist or coordinator to see if the group is a good fit for you.
Regarding how you present, don't worry about it. I joined a peer-support (i.e. no therapist) group back in July. I asked the coordinator ahead of time if it was okay that I was presenting male. She was very clear that presentation was not a factor, and that I would be welcome no matter how I was presenting. I went to my first session presenting totally masculine (I had, by that time, at least shaved off my beard!), but I introduced myself as Kathy, MTF. Everyone was fine with that, because they had all been in a similar position at one point. I now present as feminine, with makeup and wig, and more often a skirt than pants.
We have had another member join since I did, who is not comfortable yet about presenting as female. Her first time at the group, she had not picked out a name yet. We addressed her as "hey you" in fun, because we didn't want to use her deadname, and now she has a proper feminine name. She still dresses masculine, but now has a more feminine hair style.
Part of the joy of a group is seeing the visible progress that members make in the course of their transition, as well as hearing about the invisible progress.
The point is to not feel self conscious. A good group will be totally accepting of who you are and how you present.