bp_assignuser_forum

/**
 * Assign users to forums upon registration
 */
function sc_user_activate_forum( $user_id ) {
	$current_user = get_user_by( 'ID', $user_id );
	// Subscribe to forums in listed IDs
	if ( groups_is_user_member( $user_id, 7 ) ) {
		$forum_ids = array(
			997,
			1070,
			1192,
			1194,
			1196
		);

		foreach ( $forum_ids as $forum_id ) {
			bbp_add_user_subscription( $user_id, $forum_id );
		}
		// Grant access to view hidden forums
		$current_user->add_cap( 'read_hidden_forums', true );
	}
}

add_action( 'bp_core_activated_user', 'sc_user_activate_forum', 1 );