WordPress at a glance
action is not described

register_post action-hook . WP 2.1.0

Fires when submitting registration form data, before the user is created.

Usage

add_action( 'register_post', 'action_function_name_6007', 10, 3 );
function action_function_name_6007( $sanitized_user_login, $user_email, $errors ){
	// action...
}
$sanitized_user_login(string)
The submitted username after being sanitized.
$user_email(string)
The submitted email.
$errors(WP_Error)
Contains any errors with submitted username and email, e.g., an empty field, an invalid username or email, or an existing username or email.

Where the hook is called

register_new_user()
wp-includes/user.php 2393
do_action( 'register_post', $sanitized_user_login, $user_email, $errors );

Where the hook is used (in WP core)

Does not used.