Install Visual Studio Code. It is free and has Win/Mac/Linux versions.
Next install the Live Share Extension Pack. This includes the Live Share plugin and the Live Share Audio and Chat plugins.
Once these are installed, read the quickstart for joining a live share.
If you want, read the longer documentation about all you can share (editors, shells, servers, etc.) during the live share.
During the Dev Congress, one person can create the live share, start an audio call, send the live share link on slack, and then others will join. The host doesn't need to be the person leading the swarm but should