For the groups I have been in (one for 10+ years) you will be glad to have someone else joining at the same time. I don't know if you ever started a school year "late" but it is sort of like that, you aren't quite sure what is going on or who the people are and the dynamics, etc. so being able to have someone else to share the others wondering about you all at once is a bit comforting.

Sessions are never wholly about you, you don't "star" or anything, you sit around and talk seriously, as in regular face-to-face therapy but other members respond to you rather than the T so much. I guess it is kind of like here/an online forum, but face-to-face. The sessions last longer than regular T though so you talk about something and then it morphs into someone else's issue brought up by the conversation and then on to another person, etc.