In Joint Hunt, you can challenge powerful monster leaders with other players, and earn Gear Renovation items as rewards.
Simply tap the Activity Overview on the top right, and select Abandoned Alchemy Lab under the Get Stronger tab as illustrated below:

This will bring up the details of Joint Hunt. Tap Go to proceed.

Defeat a Boss for the first time to earn First Time Completion Reward.

Defeat a Boss under the current difficulty unlocks a higher difficulty which can be selected from the top right part of the menu.

Joint Hunt requires a minimum team of 2 players to enter. You may join an existing team, auto match for players or create your own team.