When you delete a user, SMS Central does not wipe them off the face of the Earth (or our app). More so, we "archive" them and mark them as "deleted"
If you accidentally deleted a user, you can easily restore him/her as an active one.
To do so, go to Account > Users and click on Show Deleted Users.
Select the users you'd like to restore.
Finally click on restore at the bottom of the table.