You've got TWO handy options for adding new users on your Group, which you can access from your Dashboard:
The Add People link will allow you to add users who are already in the New/Mode system. You will need the Username of the person you are adding to accomplish this:
The Invite People link will allow you to invite new, unregistered members to your Group. You will need to create a Username and enter the new user’s email address. Your new group member will receive an invitation to join your organization:
Learn more about managing users on your Group here.