If you have deleted a client but want to re-active their account, go to the Client Index page and click on the All Clients drop-down.
Select Disabled Accounts.
Check the box next to the client you want to re-activate and use the bulk action drop-down to select Reactivate Account.
This client will now show up on your Client Index page. All their past data, messages and check-ins are still saved on their account.