Good idea, Chr0nos, Amaunator43, marajango, HunterRayder93
However that is a complex issue and game breaking. i think it would easy to implement, the problem lies with story telling. if it was to be added... there should be an additional check to see if the NPC being spoken to is receptive to group conversation or if they would rather talk to the party leader only. that has to be added or else you've borked the entire story. not to mention if you realistically carried on a conversation with one person as a group of strangers interrogating them for information out of the blue... they would be intimidated and possibly stop talking all together.
[edited] i didn't provide a solution....
Just do a simple intimidation check on each person jumping into the conversation, so it be comes a do at your own risk scenario.