Collection of Users associated to a User Group - Custom Notify Field

Use Case:

As you know I'm building our Change Request proces using various Custom Actions. Now I have been asked that select users should have emails sent once the Request hits the next stage. The requirement is:

a) Each STAGE has a User Group with multiple users assigned. 

b) Not all the users should be notified. We have Non-Notified users who can approve Requests if people are on holiday (We use User Groups as Custom Action Security).

c) User should be configurable PER Group. Example can be notified by email in one group but not in another.

So i was thinking of:

i) To fulfil c) above - I presume i would need to add a field called Change Request Notify toggle* to **All Links->Membership->User Group Hierarchy->Group Member*?

ii) When using the Send Email in the Custom Action, i know in the Formula part you can build up who to send the email to. Now i want to send the email to a collection of users who are part of the assigned group where the Assigned Users Group Member field Change Request Notify = TRUE. how do i iterate through the container of users for that assigned group?

PS - I hate your collection handling in custom actions - PLEASE GIVE ME A FOREACH ITERATING FUNCTIONALITY! :)