Guys, I could really use your help on this one...

My fraternity was throwing a party tonight; there was a beer pong tournament and some dancing. Every guy playing had to have a partner, so I invited a girl I had met at a party and had dinner with a few times. Later that night, I found out that we had a bunch of girls from an all-girl school coming over to join us. As the girls arrive, I see this one stunning girl walk in an amazing dress; easily an HB9. She arrives with about three other friends and they sit on the couches in the corner.

I head over to the set of four girls: our HB9, an HB7.5, and then two HB5s. They were just talking to themselves and hadn't been approached all night. I open the set and get very warm responses from all of them. We continue talking and "it comes up" that I sing and play the guitar. HB9, begs me to get a guitar and play for them, and after some banter, I agree to play for them.

Just as I do this, my beer pong partner, the girl I invited, comes up and taps me on the shoulder to say hi. It's my turn to play, so I tell the set that I'll catch up with them after I finish my game.

While I'm playing, I'm flirting with my partner and such while she's looking, but the whole time I'm making eyes across the room with the HB9 who's still sitting with her friends. I lose the game and then introduce my partner to a friend of mine. I get them talking, say I have to go talk with some people, and then I run off and head back to the set.

After some more talking with the set, they agree to head downstairs to our basement where there is music, a black light, and a laser light display. The HB9 and I continue talking, all the while I'm getting several IOIs. She asks me if I could still play the guitar for her and her friends. There aren't that many people downstairs right now, so they turn off the music and I pick up my guitar.

I play some songs and the set is totally into it. By this point, I had already had a couple drinks in me, but HB9 was the group's designated driver, so she was not drinking at all. After I finish playing the guitar, we put the music back on. A slower song comes on and the HB9 and I start dancing. We're gazing into each other's eyes, and I can tell that it's the time to kiss her. Suddenly, I feel a tap on my shoulder; it's my friend, saying he has something urgent to tell me. I tell the HB9 that I'll be right back and that I had to deal with some frat business.

We walk out of the room and my friend tells me that he had been flirting with another girl earlier tonight when he saw the group of girls with my HB9 walk in. He noticed the HB9 and made the comment to the girl he was with that "I know [my name]'s taste, and that's exactly the kind of girl he'd go for." Turns out, the girl he was with knew her. She tells my friend that HB9 has a boyfriend, but "the boyfriend is hideous."

I return back to the set, but I'm torn. She's talking with her friends, and I enter the conversation and start leading it. I engage the entire set this time, focusing my attention on all of the girls rather than just the HB9. It's around this time that the party is winding down. I ask the HB9 and her friends if they'll be coming back soon, and they say that they'd love to come back, they just need to know when our next party is. I get the HB9's number and tell her I'd inform her of any upcoming events and she smiles warmly at me. I walk the group back to their parking spot, and some drunk follows us, asking if the girls can give him a ride. I get him to back off, help him find a bus, and then wish the girls good bye.

I head back to my dorm and get a text from the the girl I invited to the party and turns out she wasn't exactly happy about me ditching her, since she "came for only one reason," and that was "to see me."

So guys...what the fuck am I supposed to do about this?

TL;DR? I met this really hot girl. We're about to hook up when I find out from a third party she has a boyfriend that she told me nothing about. In meeting this hot girl, I also happened to ditch another girl who I invited to the party that came to see me. She texts me afterwards and is pissed.