When this happens, I usually try to befriend him on some level. It's hard for people to act like a jerk if you have a strong frame (you won't get a strong frame just from practicing pick up lines, but you will get one from being as social as possible). I try to smile at them, listen to them remain in control. Smiling at them is key as it shows you are not intimidated by them or worried about their presence.
Who knows, they may even be cool and add to the social situation. If they are cool, that makes you look cool also as you are now hanging out with ANOTHER cool person (you were with cool people already).
If they are really a jerk, and try to make fun of you or say something stupid I will either just ignore them, act like I didn't hear them. And sometimes introduce them to the group as the coolest random guy I have ever met in the last 5 minutes or something funny. The key is to remain cool and bust on him a little bit. Once again, you almost never have to do this as if you are nice to someone up front - they are cool back - this is only for people who act like jerks and try to AMOG you (I hate that term - but it is descriptive for this case).
I know when I was first learning, it was scary - but the more you deal with this (even the frustration and failures), the better you will get.
Go for it!
Bill