Utilizing Resource Loads

We'd like to start utilizing resource loads, however we are struggling with how to do so. The task could have multiple people working on the task, even at the same time, which if all of the users that could end up working on the task are assigned to the task, it throws off the resource load. We've contemplated having the manager add the resource who will need to work on the task each time before the task begins, but I guess this could become tedious in our manufacturing environment if the manager must leave the production floor to go and assign each person to the task when they will be working on it. So I'm looking for the best way to utilize the resource load when multiple people may be assigned/need to work on a task depending on the day.