You can quickly add existing (along with any new users) to a specific group by using a CSV import.

To do so:

•  select "Manage > Users and Groups" from the top menu
•  select the group you wish to add users to from the Groups panel.
•  select the "IMPORT USERS" button to add your CSV file. 

Users will be imported into the selected group as well as the "All Users" group.

To learn more about how to set up your CSV file see our article on Inviting multiple users.