It's as easy as using WhatsApp or Signal, but you don't use a phone number. You put a nickname and little else. 

To add contacts you have to scan a qr and to enter groups you have a search engine.

Install it and experiment. 

 I use SimpleX to talk with my family. It was fucking hard to convince them all, but it was worth it, now they got used to it. 

 Wow. I'm impressed.  My family gets mad at me if I ask them to download an app.