Hi @hyudolee Currently this is difficult as the UX doesn't support a filtered list of Users. We can do this though by exporting the Users list (or exporting from a filtered saved view) and importing to another list. That list is either a pre-filtered list of managers or you can put a subset on it. Create a module dimensioned by that new list or subset to use with the notification action.
I've created some screenshots to help here:
Imported Users to a list called 'Fake Users' and set a subset for 'managers'
A module is created dimensioned by that subset with a FINDITEM on Users list in a Users formatted line item.
This gives us our filtered list of Users for use in the notification action.
We’re actually looking into this at the moment as part of some extensions to support notifying multiple recipients driven by a module. One of the module examples we’d like to support matches this idea quite closely with scenarios like “send a notification to all users in my selected region where the ‘forecast completed’ boolean is false”.