Kodisync
This is a hacky Node script which attempts to sync playback of two or more Kodi instances.
It will wait until all Kodi instances have the same TV show episode loaded (by matching show name, season number, episode number) and then pause everybody's playback and seek them all to the earliest position among the viewers.
From there it checks intermittently for state changes (play, pause, seek), and attempts to sync them to the other instances.
Status
Rough around the edges.
The main issue is that without any kind of event-driven API we have to resort to polling.
Operation
When any instance seeks while playing or presses play from a paused state, we pause all instances temporarily, including the one where the action took place. The other instances are then told to seek to that position. However, seeking in Kodi seems to be inaccurate even when two instances are playing exactly the same file, and so they may not be in exactly the same positions. To mitigate this, the commands to play are staggered based on the newly-reported position of each instance.
When an instance is paused or seeks while paused, all other instances are told to pause and seek to that position.
Requirements
It expects to talk to each Kodi instance's JSON-RPC server, so each Kodi instance must have this enabled and you must have network access to it.
In Kodi you should only need to switch on settings → service settings → control → allow remote control via HTTP; no need for "application control".
You could set up an SSH tunnel or ngrok if you need to get through firewalls or similar.
As for dependencies, you can install them with npm install.
Running
Invoke the script and give the hosts (and ports if not 8080) of the Kodi servers as arguments.
node kodisync.js localhost my.friends.server:1234
You can also use Docker:
sudo docker build -t kodisync .
sudo docker run --rm -it --init kodisync 172.17.0.1 my.friends.server:1234 #172.17.0.1 is the loopback IP address of docker.
It will automatically add http:// if no protocol is given,
and will add /jsonrpc if no path is given.