It seems that whenever I write a "Basics of Entertaining" post, some really great questions pop up in the comments. Thanks! I love questions! The only bad thing is that they often get missed by everyone and chances are if one person has asked this question, then someone else has, too. I've decided to start featuring some of my faves.
What is the best way to mingle different groups at a party?
As the host, you are the social glue that brings everyone together and as such, it's your responsibility to introduce people with similar interests and tastes. If your space is not too big and your music is not too loud, it's easy to get people to talk and mingle. It's your job to know a little something about all your guests (that includes guests-of-guests...spend a few minutes talking to them), and then play matchmaker. If your college friend Kathy just got back from Japan and you know that your coworker Steve will be going next month, introduce them! Are Mike and John both in IT consulting? Hook them up! Are Pat and Chris both single and like to camp? You know what to do....
Now, there is protocol on how to properly introduce people, but when you're in a casual situation, generally it's enough to say "Kathy, this is Steve" at which point they'll shake and introduce themselves and then you say "Kathy just got back from three weeks in Japan". Hopefully Steve will say "Oh, I'm going over there next month!" and they'll take it from there. You can hang out for another minute, and then excuse yourself. Soon Kathy and Steve's friends will join their conversation and before you know it, your coworkers and your college friends are chatting. Possibly even about you.
The above advice works for people of normal social extroversion. If someone is painfully shy, you might have to stick around for a few minutes and facilitate conversation. Once they're doing more talking than you, excuse yourself. It's important for the host to keep circulating and not stick with one group for too long.