How to apply Default Members Availability(%) for New Project of User Group to its Members ?

Clarizen provides several fields related to User Availability(%)

1. User field: Default Members Availability(%) for New Project
   
2. User field: Maximum Project Availability
3. User group field: Default Members Availability(%) for New Project

I would like to create a workflow rule, to make "whenever the value of Default members availability is modified in the user group, apply the same value to all the Members of the user group." come true. 

What should I do?

Should I create this rule under user or user group?

Is there any function I can use to get all the users of the group?

Thank you in advance.