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 already have a checkout of the code and the test suite passing locally. The host should have a stable network connection to avoid others being kicked off the live share session.
Note that regardless of who makes code changes they will appear in any commits as the host of the live share so the host should always use co-authored-by in the commit messages to credit all who worked on the commit.