WordPress at a glance
action is not described

before_signup_form action-hook . WP 3.0.0

Fires before the site sign-up form.

Usage

add_action( 'before_signup_form', 'action_function_name_8550' );
function action_function_name_8550(){
	// action...
}

Where the hook is called

wpmu_signup_stylesheet()
wp-signup.php 86
do_action( 'before_signup_form' );

Where the hook is used (in WP core)

Does not used.