Assign ressource

Sorry, but I really still dont't undesrstand the need.

I understand your request, but I would like to be sure to is facing a real need

In my opinion, you should not assign many resources to single task.
If you do this, possibly it is to define a team, and you still don't know who will work on the activity. Then, maybe you need to define an abstract resource with capacity of 5 or 8 to plan the activity. The resource who really works on it will just as to be defined as responsible to be automatically assigned.