Never went through any kind of formal screening, but I am sure my therapist only asked me into the group because he felt I would do well in it and be able to utilize the group and function well within the group.

I wouldn't worry too much about it. It is probably just a fairly informal interview process. Unless they see any big red flags like you seem unwilling to follow the rules for the group (confidentiality, taking turns, fair interaction with others, such severe issues that cannot be addressed with in a group setting, etc.), it will most likely be no problem.