Note: This is an administrator-only feature.
When you have opened Organization Settings, go to the "User Management" and then to the "Deactivated Users" tab. All the deactivated users in your account will be shown in this tab. Locate the user that you would like to re-eactivate, scroll to the three dots on the right-hand side and select " Activate Account".
The user will get access to all the workspaces they were part of and to the account immediately. This will not trigger an email to the user who just got reactivated.