There are several options for a user to activate their own account, depending on the option used by an administrator to create user accounts.
If an account was created for them by an administrator they will receive an email notification containing their login data. After logging in for the first time they will be free to change their admin generated password.
If an administrator invites / imports the user they will receive an email notification prompting them to click a link in order to set up their own account.
If the login page registration option is turned on by an administrator, users can navigate there and create their own accounts by clicking on the Create a new account button.