You can find the Task reservation setting in the co-op editing window. Here, leaders and co-leaders can give other players the right to reserve tasks.
To reserve a task, tap the yellow button next to the Start button in the task description. An exclamation mark will appear on the reserved task's icon and will be visible to all co-op members. The reservation will be in effect for a limited time. To see how much time is left in a reservation, go to the task's description and tap on the picture of the player who made the reservation. The reservation is lifted when the time is up.
You can only reserve one task at a time, but you can change the reservation to a different task, in which case the previous reservation will be automatically canceled. Remember that if you cancel your task reservation or if the reservation time expires, there is a waiting period before you can make another reservation.
Reserved tasks can be deleted by leaders, co-leaders, and elders. The reservation can also be changed by players who have been given the right to do so.