How to send email to nextcloud user?

In the nextcloud email client I can address groups from the user database as target for emails. But I can not pick single users from there.
I other words: I need to use the user list as email addressbook.
As admin of a managed NC instance I have no access to any command line interface. And I have no chance to use an LDAP workaround because the lack of an LDAP service.
Any idea welcome to get the job done