I'm not saying they should barge in, it'd simply show up as a choice in the dialogue responses that you can select (or not).
I mean in real life when you're talking in a group you don't have just 1 on 1 conversations while everyone else in your group just stands idly there unable to say anything or talk to the person, if they hear something they know about or have some insight they tend to offer it.
It makes little sense that a character that is say, being searched for by the Red Prince and then you start a conversation with that npc w/your main character selected and they bring him up, while he's literally in your party and standing there and there's no way for him to come into the conversation in response to that, it feels a bit weird.
In real life if someone was talking to you and a group of your friends and then they mentioned one of your friends who's standing right there by name and commented about them that friend would logically say something back, not just act as though they aren't standing right there.