I kind of feel bad if I get up to leave and I don't want it to be the cue for EVERYONE to leave if they are all in the middle of a good conversation, but it always seems to happen. I usually just make leaving moves, like going to the kitchen, getting my coat, and then I might break in and say, "Sorry can I just break in for a second? I'd love to stick around and hear the rest, but I have to get going."
Unfortunately I think at least once that was taken to mean I was bored. My two companions were talking animatedly about some subject and I was very interested, but I didn't know enough about the topic to contribute. But I was enjoying hearing the conversation. But then, I really did get very tired. It was well past my bed time and I'd had a long day; I was fighting to stay awake. And I had a similar problem in that the conversation was so animated that I didn't know how to break in. Finally I did, and they were all apologetic for talking so much about X, despite my reassurances that I was interested. And one of them started asking me questions about something he knew I did/knew about, and I am sure that was to make up for his perception that he'd somehow unfairly dominated the conversation. I answered the questions about my thing to some extent and then said, "I'm sorry, I'm really tired, but I don't want you guys to feel you have to leave too; I'll just slip out but please, keep on talking." (Because one was staying with me, I lived a block away, and this person had a key anyway so could get in without me).
Apparently after I left they continued to talk about Topic X for a couple of hours, which was what I wanted them to do, not leave because of me!!