autosubscribe_forum.php

add_action( 'wpmu_new_user', function( $user_id ) {
	add_user_to_blog( 19, $user_id, 'participant' ); // add user to forum site as participant

	// subscribe user to forum
	$forum_id = 0; // update the forum id
	switch_to_blog( 19 ); // switch to the correct site
	bbp_add_user_forum_subscription( $user_id, $forum_id ); // add user to the forum
	restore_current_blog(); // switch back to the original site
} );