For example, consider the example OU of Test, only for whom you want the backup to run.
Go to Backup> Settings> Backup settings and switch OFF 'Add new Domain users'. -- this will disable auto-backup at domain-level, so that no new user added to the domain is backed-up
Go to Backup> Settings> Manage Users, and go to Org Units.
For every OU except Test, switch OFF auto-backup. -- this will disable auto-backup at OU level so that no new user added to the OU is backed-up
For every OU except Test, pause the backup (button is available under actions) -- this will disable backup for already archived users in the OU.
Now, only users under Test OU will be actively backed-up. Also, only the new users added in the Test OU will be auto-backed up into the application. If you move users out of the Test OU, the backup will stop for them. If you move users into the Test OU, backup will resume for them.