Executing a Rule on a user(s) selected from a list
It sounds to me like you need to fetch the user as an entity in Rules, if you have multiple users use a loop. After this you should be able to apply the relevant action. Here is a stab at the Rule (note, this is untested but should point you in the right direction):

Event 
- (Whatever trigger/s you want here)

Condition
- Entity is of type: Node
- Entity has field: (pick the user reference field here)

Action
- Loop(for multiple users)
- (Final action you want to take place. Add the new variable here)

Hope that makes sense. See how you go with this and let me know.