Someone who accompanies you or your group. A member of your posse. There can be a negative connotation, as in someone unwanted, but that is often not the case.
Person 1 "I'm going to get some pizza, anyone want to come?"
Person 2 "I'll be a tag-along"
Some that tag's along with you everywhere and invites themselves over to your house, some what like a douche bag
Tyler why are you inviting yourself over to my house cant you jump of a bridge like everyone else
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.
Someone that nobody likes that follows you around
Jackson is a tag-along
1: A gift parasite, someone who writes their name on someone else's gift tag or card. They take credit for someone else's gift so they don't look like the cheap person they are. Either that or they're broke but don't want anyone to know.
2: An annoying person (usually a younger sibling) who, for some reason, wants to go everywhere you do.
1: Bob: Dude, I just got fired. On the week of Christmas too!
Joe: Well, Suzy's Christmas party is around the corner. What are you gonna do?
Bob: I think I'll just tag-along on Billy's gift.
2: Guy: Bye mom I'm going to the movies!
Little sis: Why can't I go?
Mom: Take your little sister, Guy!
Guy: Aw mom, she's such a tag-along!