I think it's also dominate behavior, to try to see who's who in the new pack. At least Rudy's jumping on Joey's back sounds like it to me. I would interrupt and stop that behavior. Others more experienced will have more to offer than me. Observe how Joey is responding and handling the new arrival. If it appears Joey could use your help to teach Rudy some manners and respect, I suggest you step in. Rudy probably needs some guidance on boundaries and what's expected and allowed.
Invitation to play is usually easy to spot with a play bow, pawing, a bark or growl. You may have a mix of play ad dominate behavior going on. Be sure to look out that Joey isn't getting picked on. Let Rudy know what is acceptable and is not. A container of coins or water spray bottle can help interrupt behavior you want to discourage. Also vocal, as in hey, or uh-uh!
Cindy