Allow Joining Player on Adventure Island

[AntumDeluge]

💡 Describe your idea

We could allow players to join another player already on Adventure Island.

• Fee could be half of normal fee of player asking to join.
• Should not count towards player's quest (e.g., player can still ask to enter for their own island).
• Either, the island owner must confirm that that they want the other to join or, they must be required to be in a group (might also require that island owner is leader).
• This would also mitigate the issue sometimes hard to find enemies for Daily Monster Quest.

There may be caveats to this idea, but I haven't thought of any yet.