You must have an account to administer only your Zentyal server.
Then you can create other accounts which can use the services and LDAP. You will then have 2 accounts minimum : one for Zentyal Server, one for using it.
The accounts you create are for using samba shares, ftp, mail, etc... in a windows-like domain environnement with LDAP. Do not confuse with the linux system users and groups which are only used for/by the system.
Unless you tick the case "give bash to LDAP users", Zentyal/LDAP users cannot log/ssh to the server. They can only log to the domain.