Ok, next time add yourself to the conversation like you said, if you have a target just figure out how you can keep adding yourself to the conversation, dont go down without a fight lol.
Being a good conversationalist can help in this scenario.
Here's what i would have done
Im in 2 set, guy walks in, starts talking to my target who's on the other side.
Immediately my ears perk up, i listen, and i include myself in the conversation + include the friend + include whoever is around, then i push the conversation into isolation between me and my target, very smoothly transition to a personal question and position myself next to my target, now depending on how good he is, im assuming his an average guy with no game, he'll just move over and talk to the other girl. And there you go. You've won the attention battle, now build attraction + comfort and make the magic happen.
And yea, in your situation, when the girl went to the bathroom, i would have just weaseled my way into the other dudes conversation(this takes social calibration) very smoothly, and done what i said above
Dont get stuck with the girl you dont like, manipulate conversations in your favor, that dude was an asshole
but he outplayed you.
And about the AMOG, no worries, dudes love to amog, you have to be bulletproof.
I'm not going to go into details here about how to AMOG/Handle amog but ill make a video about that soon because i personally love to AMOG/hangout with alot of AMOGS and i know quite a thing or two about that.
but my advice for amog, *ignore em* focus on target and good vibes, dont let em get to ya
Good luck