Your license with Zahara is for named users. This means if you need to register a new user, you need to make sure your license count will allow for a new user. If you are taking a trial, the you have unlimited users for the first 30 days so add as many as you require.
To add a new user, and to see the Admin > Settings menu, you need to be a tenancy admin. This is enabled on a user by user basis under the users main settings.