Hmm....I don't know. It's possible that 'rubbed off' on that other person, but it's also possible that it had nothing to do with you. Hard to tell for sure.

It sounds like the group is helping somewhat, especially the facilitator. Does that facilitator have any other BPD groups you could attend?

Maybe hold off on talking about your SI at group for a few weeks, and just discuss that in your individual therapy sessions.