Tag along (used as a verb, adjective or noun)
Situation #1: Two friends
Kate: I'm so excited. I'm going to check out that new horror movie!
Chris: Really? I've been wanting to see that. Mind if I tag along?
Kate: Not at all! Let's go!
Explanation: "Tag along" means to go with someone. When used as a verb, it's usually asked in a question (as in the above example, or if Kate had asked "Would you like to tag along?"). However, when used as an adjective or noun, "tag along" (or sometimes "tagalong" or "tag-along") is usually more negative, suggesting that the person is following another in a constant and possibly annoying way, as seen in the next example:
Situation #2: Two friends
Brie: Have you seen Valerie?
Kim: No, why?
Brie: Well, I'm avoiding her. She is such a tagalong! She follows me everywhere, and it's getting annoying!
Kim: She doesn't seem like she's that annoying.
Brie: She followed me into the bathroom earlier, even though she didn't have to go, and then she followed me to my class, even though her class was on the other side of the building! I don't need a tag-along friend!
Kim: Uh...