My dad is a postman, and he talks about me with his customers. I've had people I don't recognise wave to me, or ask me how I'm doing, as they know me through association with him. It's a bit disconcerting sometimes, as I feel like I ought to know the person in question, but I've gotten used to it. I'm not overly keen on talking to my parent's friends for an extended period of time, but exchanging pleasantries isn't difficult, doesn't cost anything, and it's quite nice to know that, even if it's just through association, they're interested in my wellbeing. There are plenty of other things they could talk to my dad about, so that I'm a topic of conversation at all is actually quite flattering.
If people come in the house, I usually just say hi and then go upstairs out the way, because I have no interest in the conversation and/or wouldn't have anything to add to it. It's common courtesy to greet a guest, even if you have nothing to do with them, but beyond that I prefer to give my parents some privacy...they did it enough for me when I had friends over when I was younger, after all.