Got something very similar coming up myself soon, only this is a new years night out, the one night on the year I hate going out... but that's by the by.
Sounds like you handled meeting the other guy from her work well. Experiences like that are never nice but they are a good opportunity for us to show how secure and unaffected we are.
This is probably the best way to view these upcoming parties. Being the outsider in somebody elses group of friends can be unpleasant. But it really is a fantastic way to show that you're not needy and perfectly secure. Go along to the party, avoid sticking to her and make the goal of the night to get to know and chat to some new people. She's probably going to want to run around and talk to everyone, so relax, let her do that, ignore any flirting. Show her just how unphased you are with the situation.
After all at the end of the night she's going back with you.
