Adding a plan to a group calendar makes that plan available to everyone in the group, but it won't automatically apply it to their own calendars (unless it is a Default Plan). That's where Group Plan Subscriptions come in.
You can set up Group Plan Subscriptions individually, or you can subscribe all group members (not including group coaches and admins) in one click. To do this:
Navigate to the Group Profile
Locate the Plan and click "Subscribe All Members"
And that's it! The plan will now be added to every non-coach admin member's calendar, and will sync any changes you make from here on out.