I'm going to document my experience adding the session sharing plugin to nodebb here, in case it will help others. My forum is deployed to Heroku and Initially NodeBB crashed when trying to install the plugin.

1. npm i nodebb-plugin-session-sharing
2. git add *
3. git commit
4. git push -u heroku master
5. In the ACP find Plugins > Install Plugins > nodebb-plugin-session-sharing click activate
6. Click Rebuild and Restart Forum
7. App crashed

I reverted to the previous good commit. I upgraded NodeBB, then tried to install the plugin again, with success.

1. git reset --hard <last good commit> (know what you are doing before you use "reset --hard")
2. npm i nodebb-plugin-session-sharing
3. ./nodebb upgrade
4. git add *
5. git commit
6. git push -u heroku master
7. In the ACP find Plugins > Install Plugins > nodebb-plugin-session-sharing click activate
8. Click Rebuild and Restart Forum
9. Reload the page when the Rebuild and Reset is done.
10. Find Plugins > Session Sharing