Create an action with two steps.

Step 1 is from 1-1. Send an email to someone. Interval is 120

Step 2 is from 2 -0. Send an email to a group of people. Interval is 21600.

The first time an action is executed, the members of a group are cached.

If I remove or add a member to that group the action in step 2 still only sends emails to the users originally defined.