This happens to me a fair bit so I must be the problem, but anyway.
Say I am chatting to a friend/acquaintance around the town where I live, work, a coffee shop, a bar... maybe we've bumped into each other, maybe it's at the end of an exercise class, maybe we've gone for coffee...
So we are deep in conversation and another person the friend I'm with knows, happens to go by. If it's me who knows them and I am already talking with someone else, more often than not I will acknowledge the new person, try to be polite but end the interaction as soon as possible as I'm with someone else; if I feel it's going to take longer then I involve the person I was with originally.
However what happens to me a lot is that the person I am with will engage fully with the new person without involving me. Initially I try to be a part of it by smiling and laughing with them, but soon it's clear I have nothing to do with whatever they're talking about and maintaining that position is exhausting, so I'll stop doing that and then feel uncomfortable because I've just been left hanging.
There's a couple of women at work that do it every single time so I no longer engage with them because I feel they are rude and treat me like a second rate person. Other times however, I genuinely don't know what the best thing to do would have been.
How does everyone else navigate these interactions, if you even know what I'm talking about?