How define that when one user create document, then auto subscribe on notifications for it not only user who created, but also to all user from some group?

For example model have user group USERS. We create three users that assign to this group: USER1, USER2 and USER3. And create one user USER4 that not from this group. When USER1 create new document from model, then to this document auto subscribed USER1, USER2 and USER3 because they from group USERS. When document create USER4, then again auto subscribed USER1, USER2 and USER3 to second document.

What right way define such autosubscribing for group?