That sounds really rough, and I’m sorry for that. You seem to both have different ideas about the relationship you have with each other. I’m prepared to give adding another friend to contacts after saying you have enough already a grudging pass, but in this case, not how it was actually done. She didn’t just give another guy her number in front of you, but she practically shut you out while they were talking. They are “both adults” and might not have meant anything, but her behaviour (at best) was rude and inconsiderate of you.
Have you heard of a saying that goes “the problem is not what I did, it’s your reaction to what I did”?
It probably would have been better to discuss how it made you feel, calmly and more directly later on, but I think your reaction/hurt itself is understandable. Don’t let her tell you feeling angry for getting shut out (orchestrated, at that) is the issue, rather than how she (and those other guys) treated you.