Have you thought of asking her to hold off on the physical aspect of her relationship with them for a month or something while you two have time and space to work through everything? There are pros and cons to this, but it might take some of the pressure off of both of you - you will be able to talk to her without the triggers of jealousy because she just spent the night with them, she won't have to feel defensive because she knows you are tense about it.
This can give you the chance to see if she means what she says, that you are more important, if this is something that is gnawing at you.
One concern I have though is - if you see somebody else, why does she get a don't ask don't tell policy, where you get to know who she is dating, and when she is out with them? I see that this could cause problems, especially if you do and start caring about somebody else as much as she cares about her friends. Might want to have more conversation about this at some point.
I think you're doing an awesome job of being open and supportive considering how this played out - she is very lucky. Cut yourself some slack, maybe schedule times to have conversations so they aren't going to be at horribly bad times for either of you. Keep asking for support and advice here.
Happiness will never come to those who fail to appreciate what they already have.