Tag-along
A person who follows around another particular group of friends (a "clique") but does not really interact with them. This person tries to be friends with the group, but due to any number of things, the person has nothing to share with the group or anything to add to the conversation. This creates an incredibly awkward situation where the group wants him to go away, but says nothing, and the person feels he does not belong, but doesn't want to just leave.

Therefore, the person decides to stick with the people and hope for the best. He hangs on the edge of the group and follows them from time to time, becoming a "tag-along"
Jeff: Dude, was that Mike that just walked up, said "Hi", stood around us for ten minutes, then walked away?

John: Yeah, Mike is such a Tag-along.