Add Fields TO REgistration

You can try to create a wp-idea-stream-custom.php file (see https://github.com/imath/wp-idea-stream/wiki/wp-idea-stream-custom.php for more infos) and include this filter in it.

function paul_kelly_registration_fields( $fields = array() ) {
	if ( empty( $fields ) ) {
		return $fields;
	}

	return array_merge(
		$fields,
		array(
			'first_name' => __( 'First Name' ),
			'last_name'  => __( 'Last Name' ),
		)
	);
}
add_filter( 'wp_idea_stream_user_get_signup_fields', 'paul_kelly_registration_fields', 10, 1 );