It's 'tags along'. 'Tag along' is an informal way to say 'accompany'.

If you mean "tag along," it does mean "to accompany," but it has a negative connotation to it- someone who is joining an activity, but isn't necessarily a welcome addition to the group. You can also put it together as one word: "tagalong." I've heard it used as both a noun and a verb. Examples: "I went to meet my friends at the mall yesterday, and my little brother insisted on tagging along. He is such a tagalong!"