An app for iPhone or android. Something you would find in google play or the apple App Store.
To be honest I did not ask, but assumed. I can go back and ask for sure.
From my conversations it seems as if maybe a larger percentage of people who do the retreat are more techie oriented.
In our first conversation it was mentioned several people had suggested discord. That is great if you are in a project group or gaming, etc. But for most of the world once you leave the location, life is much more asynchronous.