When you delete a user, you can delete it completely from DriveHQ.com, or remove it from your group. If you remove it from your group, the user account becomes a standalone account.

If you want to add it back, you can use "Add an Existing User" in the "Group Admin" page. Note in this case, the user must accept your invitation. (For security reason, you cannot grab a standalone user into your group without his approval).