Possibly you could reward players for playing well with others. An example of this mechanic would be to divide experience between players depending on the level. This would incentivise players for working with each other. This could be made more enjoyable and less grindy by beefing up the lower level monsters. These monsters would more specifically target the higher leveled players. Making it harder for the high leveled player to progress. He'll need the lower level ones to beat on the monster to keep himself from dying.

You could reward players for playing well with others. An example of this mechanic would be to divide experience between players, depending on the level. This would provide incentive for players to work with each other. This could be made more enjoyable, and less grindy, by beefing up the lower level monsters. These monsters would typically target the higher levelled players; making it harder for the high levelled player to progress. They would than need the lower level players to beat on the monster, to keep themselves from dying.