When a room or instance is full in AQ3D, additional players will be moved into the next available room. You can exit the room and try to re-enter or use a friend's Summon Word to keep attempting until a vacancy in the specific room becomes available.
In AQWorlds, you can always join a private instance with you friends. Type "/join kornconcert-XXXX" and replace the XXXX with a four-digit room number over 1000. For example, if you and your friends each type "/join kornconcert-9001" then you'll all go to the same instance of the concert.