A problem of Arelith is that often times its more profitable to solo or otherwise have the minimum number of players for a dungeon/sailing/etc. Even rune dungeons will only provide 1 drop at the end regardless if its 1 or 10 players. The quantity of mobs that spawn extra are too little to make up for it (and at times actually make it a lot harder for very little reward).
My suggestion conceptually is simple, make random named bosses more frequent when there are multiple people together. It doesn't 'freely' increase the reward, makes it a little more difficult but not unmanageable, and grants rewards in form of bounties, bigger gold drop and generic loot.
Increase the chance of one spawning in a group byy a very high margin for every extra character, maybe even two or three at a time if the parties are big enough.