a person who isn't invited into a conversation, they would usually stand on the outside of the group and just listen in, or make stupid or pointless comments throughout the conversation or tries to change the subject to something about themselves. More often than not it is someone who everyone dislikes. And everyone would generally ignore them.
(A group conversation about a Tv show)
Jim in: Omg, i had two chocolate bars today! Im so fat!
(group ignores the jim in)