Conversation Invader
A person who while not a part of a conversation will jump in uninvited. This causes the others in the conversation to give each other a "Who the fuck is this guy look."
Danny: "I can't stand that guy over there he's such a conversation invader."
Ken: The other day I was trying to flirt with this girl and he totally invaded my conversation.
Danny: What a freak.
Ken: The other day I was trying to flirt with this girl and he totally invaded my conversation.
Danny: What a freak.