Organization Owners can assign permissions to a Group. The Groups for your Organization will populate the list and permissions are applied or removed as soon as the state of the toggle is updated.
Instructions
Navigate to your organization's Notifications site (http://notifications.civicplus.com/[orgname]/admin)
Click Administration and select Permissions
Find the group you want to assign permissions to and click the Has Access toggle to the right of the group name to assign permissions
Note: The toggle is "on" when it is on the right.