From Joomla! Documentation
- Allow User Registration. Yes or No. If set Yes, users can register from the front end of the site using the Create an Account link provided on the Login module. If set to No, the "Create an Account" link will not show. Since Joomla 3.4 this option is set to No by default for new installations.
- New User Registration Group. The group that users are assigned to by default when they register on the site. Defaults to Registered.
- Guest User Group. The group that guests are assigned to. (Guests are visitors to the site who are not logged in.) This is set to Public by default. If you change this to a different group, it is possible to create content on the site that is visible to guests but not visible to logged in users. See Allowing Guest-Only Access to Menu Items and Modules.
- Send Password. If set to Yes the user's first password will be emailed to the user as part of the registration mail.
- New User Account Activation.
- None. User account will be active immediately with no action required.
- Self. User will receive an email with an activation link. The account will be activated when the user clicks the activation link.
- Administrator. User will receive an email with an activation link. When the user clicks this link, the Site Admin will be notified via email and the Site Admin needs to activate the user's account.
- Send Mail to Administrators. Send email notification to administrators with User Account Activation set to None or Self.
- Captcha. Use Captcha for User Account Registration and Username or User Password reminders.
- Frontend User Parameters. Show or Hide. If set to Show, users will be able to modify their language, editor, and help site preferences from the front end of the site. If set to Hide, the user will not be able to change these settings.
- Frontend Language. Default site language.
- Change Username. Allow user to change Username.