You literally don’t need any other mechanical change to the game to make grouping the goal of players besides a group exp bonus. Just give a group a bonus that scales depending on amount of players in the group and their respective levels. Problem solved.
The fact of the matter is that any mechanical changes you make to gameplay to try and make things harder will eventually be overcome by players to some or all extent. The simple fact will remain that solo exp is faster and so no one will group.
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