My suggestion would be to do one of two things:

#1 Forget the idea of "group sex" until you can get to the comfort zone of knowing when everything is ssaid and done, she's going home with you.

#2 If you are not in a terribly serious relationship, do the "group thing" without her.

One question though, would it bother you if she did it and you didn't see it happening? If that would be okay with you, you could do it in different rooms at the same location maybe.

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