This happened to me about a week ago, and looking back on it, I've realized that it isn't the first time. The setting is a party I went to with a couple of friends. We knew very few people there, though most of the party was composed of a fairly tightly-knit social circle.

We sort of split up shortly after arriving. I go off, talk with a couple of people I'd met maybe once or twice before, generally just lightly mingle a bit. I decide to circulate back to one of my friends to see what he's up to, and find him talking with a female acquaintance of his, whom I've never met before. He's clearly mid-story in their conversation, so I just say "Hey [friend]," as I come up to them. He almost absent-mindedly gives me a "Hey," back, and I stand there, listening to what he's saying so that I can join in at the next pause in the conversation. When this does happen, I make a joking comment on what he's said, he laughs, and we quickly banter back and forth for a couple of seconds without really thinking about it. Realizing that I've sort of cut the girl out, I turn to her and say something along the lines of "Sorry, I don't think we've met yet. I'm SergeyTuganov." She says hi and my friend does a quick introduction, but she's become quiet, and over the next couple of minutes I start to get the general vibe that she'd rather be talking to my friend, without me.

My initial approach to the two of them is sort of thing that would seem completely natural if he were talking to a mutual friend of ours, but clearly I've done something (or maybe a number of things) which had a poor impression on the girl. Should I have changed something in the way I approached (ie. completely interrupted my friend, said nothing, waited for a better time, etc.), done something different once I entered the conversation? Also, if I've accidentally made a faux-pas like this, what should I do? This is the kind of girl who I'd immediately profile as being the quiet go-home-early type, and I can tell immediately that she's probably going to routinely be a satellite for my friend throughout the night.

Any thoughts would be appreciated. I guess that in general, this question is probably about how to approach a group of people when you already know one or two people in the group, but not the rest.