For open source and freeware projects, it is very common to setup MantisBT so that users can signup for an account and get a REPORTER access by default (configurable by the $g_default_new_account_access_level configuration option). The signup process can be enabled / disabled using the $g_allow_signup configuration option, which is enabled by default.
If email notifications ($g_enable_email_notification) is set to ON, users with access level about $g_notify_new_user_created_threshold_min will get a notification that a user account has been created. Information about the user like user name, email address, IP address are included in the email notification.