For most people, I just call/SMS them with JMP, or send them an email.
If they need features for file/picture transfers and video chats, I'll have them use a free software WebRTC service.
If they need to video chat or transfer files/pictures from their handheld device which can't do WebRTC, I'll have them install Riot from F-Droid.
When using Riot, I make sure not to run it from Chromium or the Electron app, because the jury's still out on whether Chromium and Electron are free.
