groups_check_for_membership_request( int $user_id, int $group_id )

function groups_check_for_membership_request( $user_id, $group_id ) {
	$request = false;

	$user_groups = bp_get_user_groups( $user_id, array(
		'is_confirmed' => false,
		'is_banned'    => false,
		'is_admin'     => null,
		'is_mod'       => null
	) );

	if ( isset( $user_groups[ $group_id ] ) && 0 === $user_groups[ $group_id ]->inviter_id ) {
		$request = $user_groups[ $group_id ]->id;
	}

	return $request;
}