Sharepoint based websites can be configured to be password protected to restrict access to users. Only registered users can then access it and an invitation to those users is sent via email. Moreover, they also support roles, enabling administrators to restrict certain members based on their roles to certain permissions.
When adding a new user, one may store their email address and request sharepoint to send an email to the user to enable them to access the sharepoint based website. Shrepoint itself provide with web pages to administer users, user groups, roles and permissions to ensure security and ease of user management on sharepoint based websites.   _{Your Comment}