That depends on what you're planning to use them for. If all you need is a basic LDAP entry with contact information for external applications then you can create a user and deactivate »Samba account«, »Posix account« and »Kerberos principal« on the »Options« tab. I'd suggest creating a container for all those contacts and not to mix them with regular user accounts within the same container.
If you need something more specialized like a contact for the Zarafa Groupware server then you can even use a template for such entries called »Zarafa Contact« when creating a user.
Like I said, it depends on what you want to do with them later on.