[Carlo Tafuro]

Here’s my solution:

add_filter( 'wpmu_validate_user_signup', 'my_wpmu_validate_user_signup' );

function my_wpmu_validate_user_signup( $result ) {

	$underscores_error_message = __( 'Sorry, usernames may not contain the character "_"!', 'buddypress' );

	$error_messages_array = $result[ 'errors' ]->get_error_messages('user_name');

	if ( count($error_messages_array) == 1 and $error_messages_array[0] == $underscores_error_message ) {

		$result[ 'errors' ]->remove('user_name');
	}

	return $result;
}