Let me amend the statement that Mohasin made to make clear.
When a tag is applied to a monitor group it does not propagate to monitors in that group. Rather, when the monitor group tag is selected in third party integration, monitors belonging to that group alone is notified.
I believe the solution for the original request is satisfied when you apply a tag to a monitor group and use that tag as a filter source in 3rd party integration.
In the below example, 'jpjresources' is a tag for a monitor group.
Product Manager, Site24x7.