The word joke is often used with the prepositions "in" and "on," as in: He's in on the joke. Let's get him in on the joke. We're going to play a joke on him. The joke's on him. Etc. So, given this habitual usage, the writer uses "to join in on this kind of practical joke", rather than just, "to join this kind of practical joke" or "to join in this kind of practical joke" (these are grammatic