bp_messages_add_user_to_group_message_thread( $group_id,  $user_id )

function bp_messages_add_user_to_group_message_thread( $group_id, $user_id ) {

	global $wpdb, $bp, $messages_template;

	// Add Member to group messages thread.
	if ( true === bp_disable_group_messages() && bp_is_active( 'messages' ) ) {

		$group_thread = (int) groups_get_groupmeta( (int) $group_id, 'group_message_thread' );

		$recipients = BP_Messages_Thread::get_recipients_for_thread( (int) $group_thread );
		$recipients = wp_list_pluck( $recipients, 'user_id' );
		if ( $group_thread > 0 && ! in_array( (int) $user_id, $recipients, true ) ) {

			$first_message = BP_Messages_Thread::get_first_message( $group_thread );

			$message_users_ids = bp_messages_get_meta( $first_message->id, 'message_users_ids', true ); // users list
			$message_users_ids = explode( ',', $message_users_ids );
			array_push( $message_users_ids, $user_id );
			$group_name = bp_get_group_name( groups_get_group( $group_id ) );
			$text       = sprintf( __( 'Joined "%s" ', 'buddyboss' ), $group_name );

			bp_messages_update_meta( $first_message->id, 'message_users_ids', implode( ',', $message_users_ids ) );

			$wpdb->query( $wpdb->prepare( "INSERT INTO {$bp->messages->table_name_recipients} ( user_id, thread_id, unread_count ) VALUES ( %d, %d, 0 )", $user_id, $group_thread ) );

			remove_action( 'messages_message_sent', 'messages_notification_new_message', 10 );
			remove_action( 'messages_message_sent', 'bp_messages_message_sent_add_notification', 10 );
			$new_reply = messages_new_message( array(
				'thread_id'  => $group_thread,
				'sender_id'  => $user_id,
				'subject'    => '',
				'content'    => '<p> </p>',
				'date_sent'  => $date_sent = bp_core_current_time(),
				'error_type' => 'wp_error',
			) );
			add_action( 'messages_message_sent', 'messages_notification_new_message', 10 );
			add_action( 'messages_message_sent', 'bp_messages_message_sent_add_notification', 10 );
			if ( ! is_wp_error( $new_reply ) && true === is_int( ( int ) $new_reply ) ) {
				if ( bp_has_message_threads( array( 'include' => $new_reply ) ) ) {
					while ( bp_message_threads() ) {
						bp_message_thread();
						$last_message_id = (int) $messages_template->thread->last_message_id;
						bp_messages_update_meta( $last_message_id, 'group_message_group_joined', 'yes' );
						bp_messages_update_meta( $last_message_id, 'group_id', $group_id );
					}
				}
			}
		}
	}

}