YTA, but this will probably be controversial.
I know the title sounds like I’m not the asshole
Honestly I thought the opposite.
You're acting like you're entitled and get to control your friends and their time.
Fast forward a few months and I see through snapchat stories that one of my best friends is hanging out with my ex and her friends. Of course, i was confused, and a little hurt
Just because you dated her doesn't mean you get to control what your friends are doing with her. They're not even dating her, they're just friends. You're not hurt because they're hanging out without you, you're upset because they're hanging out with her. You do not control your friends, or her.
I said that I was conflicted on “our new friends” and he suggested that I stay home if I dont like it
You said at this point it's been a few months past the breakup. Look, I get that you were heartbroken, but the relationship was only a few months long. Not to say you didn't care for her, but come on. Once again, see point above. If you can't handle it, don't go. Your friends aren't required to change their habits because of you. Again, if they were dating or flirting with each other all the time, I'd say it's a different story. They're just friends.
i claimed it was really inconsiderate of her (the exact quote was “major dick move”) to get a job when she knew I worked there
At this point, I'd just say you're being unprofessional. It's a place of work. She's not working there to flirt with you and your friends. You're not the boss of the business. If you don't like it, again, leave. You working there is a privilege, not a right.
Aita for being upset with my friends for hanging out with the girl who utterly broke me?
At this point, yes. They didn't call her the day after the breakup asking her to hangout, and they're not in intimate relationships with her either. This also wasn't some lengthy relationship where you guys were about to get engaged and/or married. You don't have to want to hang out with her, but don't act like your friends are the bad guys. It sounds like they waited a few months then went back to hanging out. You guys were friends before, so they just resumed the friendship.
And am in in the wrong for thinking its really fucked up of her to work where she knows I work?
Yes, because she probably thinks that the two of you can work in a professional environment together. Not to eventually date again, to have some fun with friends and make some money. Clearly you can't, though.