This action runs just after the user is created after the submission of the registration form.
$post is an array of data associated with the user.
$files an array of files data to save for a user.
$gid is the group ID for which the registration form has been submitted.
$server is an array of data of a server’s details.
$fields is an array of field data associated with a group.
$user_id is the ID of the user that has just been created.