Go to administer, User Management, Add and fill in the form.
username: use the normal name with capitals and spaces.
password: use an initial password like "welc0me", and advise the users to change it into their own password. If they lose their password, they can contact you and you will have to create a new initial password and send that to the user.
Status: should be active, unless you don't want that user to use the content.
Assignable roles: the user gets automatically the role of authenticated user: read/not write. If you want them to be able to write assign contributor, if you want them to be able to edit/delete, assign as editor, and if you want someone to have total rights to this site, assign as maintainer.
Check "Notify user of new account" so that an email is sent to the new user. Forget about locality and scroll down. Fill in title, given name(s) and familyname. After that go completely down and click on "create new account" button. All fields not entered can be entered by the users if they like.