To call in a friend for Colosseum PvP, use a group password. Once inside the Colosseum PvP menu, the user must choose a password under the "Colosseum Password" option. Once set, share this password with your friend, and enter matchmaking. This will put both you and your friends into the match. Do remember that you and your friend need to search for a game from the same place, so if your friend goes and searches for a game in the Caelid Colosseum, he won't match you.