You can add a group by clicking on the button add. Search and select the proper group. Select the group you would like to add and submit.
A practical feature is that you can add (Azure AD) groups to your favorites. These favorites are available throughout your CMS. You only have to select your favorites once in order to assign permissions easily.
To select a favorite you click at the end of the group name.
Make a favorite
To see all your favorites click the button with the heart (green below).
See your favorites