My adventurer on Dwilight is currently travelling around hunting monsters with a friend of his, another adventurer. We occasionally come upon a group of monsters (usually with an alpha) that neither of us can handle and we thought what a good idea it would be if adventurers could hunt together, as in two or maybe three adventurers taking on the same group at the same time.
How it would work mechanics-wise...well, there are a number of ways. Perhaps it would be like a unique item trade or a duel, only instead of "Gertrude has challenged you to a duel of honour till surrender" it would be "Gertrude wishes to hunt group X of some monsters with you. Do you accept?" And if you accept, assuming you both have enough hours, you would take on the group with an increased chance of success due to there being two of you.
Another way could be the ability to make other adventurers your "buddy". If both adventurers agree, then all hunts conducted by either buddy, as long as they're in the same region together and they have enough hours, would be done together. Being buddies would be permanent until one adventurer manually ends the partnership.
Rewards would be split, with unique items handed out randomly (the winner could always just give it to his partner if he wanted).