If you deleted a user by clicking the Delete button on the Users page, it was just a soft delete. Put simply, this means that the user's email address is still present in the Award Force database but they have been removed from your user list. Because an email can be registered only once, the address exists in Award Force and cannot be re-added as a new user.
You can, however, undelete the user by following the steps below.
Undelete a user
From the Manage workspace, go to Users
Open the All but deleted toggle and select Deleted
Select the checkbox associated with the user(s) you would like to reinstate
To return to the initial view, open the Deleted toggle and select All but deleted
Permanently delete a user
If you want to permanently delete a user, you'll need to click on their name in Users and then click the Delete this user permanently button.