You mentioned feeling like you were possibly stepping on professionals toes...or get that feeling. Do you have resources for members who need a therapist or more direct help? If this isn't group therapy, you aren't taking over their role. But really, sometimes it takes a friend or peer to encourage someone to get more help when needed.
I have not done a support group for PTSD but have been in groups for eating disorders (was disorganized and confusing, so I quit) and continue to go to AA (enough structure and yet flexible support to make it helpful for me). In AA we are clear about not solving all problems. Helpful members aren't afraid to encourage more help from mental health services where needed. So, you could have a couple pamphlets or info with emergency #s or clinic/psych services available.
If just a hang out, I wonder if it might not feel very support (though hanging out is important, a big piece of trauma recovery is being able to reconnect with others). I am NOT very social. I need a little structure in order to even come out of my shell. With AA it was easy to have a meeting, then warm up to the idea of sticking around for coffee vs always running out the door afterwards. But if it had just been a meet-up/hang-out I would not have gone. Just my perspective, though I know there are many of us like this.
Could you work on a loose structure that focuses perhaps more on symptoms than trauma stories, so people can share what is happening and ask for ideas/support to get through their current situation? Or ideas along those lines. Or pick a theme and find a bit to read (like "anger" or "self care" or something....an would give structure, even if some people just need to talk about their present reality, whatever it is). In AA, some groups inadvertently encourage "My life sucks" fests. Newcomers leave. No hope there. But if there are a few members who are in a stronger place and can offer ideas of what has been helping them, then their is some hope. Hope is HUGE. But keep it clear that you can only offer peer support and anyone who needs more help or is a risk to themselves or others can be supported in finding more resources. ???
The social connection is a great idea. If you can do a group with a little structure, can you follow it up with some light social time like coffee where you are at or invitation to meet up afterwards at a restaurant or coffee shop? The social period after support could diffuse some stress and foster the positive feelings and connection.