Try create a resource with its own calendar and a max capacity of 400%, 500% or whatever.
This is the way several pools of resources work in Project. For example, you can make a resource called "Testers", value 500% for 5 testers. Then, you can assign your task to that pool.
A pool of resources is when all resources in a pool have the same calendar and the same cost. If not, you don't have a pool, because you have a differente restriction in your project for each member. You need a cost and time baseline.