For all Domino deployments that use LDAP/SSO authentication, there is no manual way to add a user. For a user account to be created on the Domino deployment all the user needs to do is login and their account will be created automatically.
For deployments not using SSO/LDAP you will see the follow message at the bottom of the login screen, Don't have an account yet? Sign up (see below). Follow the link and fill out the user information and the account will be created.
It is possible to disable signups via a feature flag, so check the Admin>>Advanced>>Feature Flags menu to enable or disable signups via the "SignupsEnabled" flag.