Project Website:https://getsession.org/
Session is a secure and anonymous communication app which is decentralized and censorship resistant. It supports group chats, communities (must be hosted on a server, only needed for very large groups), and picture and video transfer. It is available for Android, IOS, Linux, Mac, and Windows.
Session has an excellent setup guide here.
The desktop application is very similar, however, one can link devices to receive and send messages on both. Either one can be configured first: neither requires any identifying information. In some cases the desktop application cannot be “linked,” and instead you must copy your seed recovery phrase from the mobile app into Session Desktop. The effect is the same: both devices will send and receive on that “account.”
If possible, save a copy of your recovery phrase in a very secure location. If you don't, and you loose all of your devices, you can't recover your account.
Session Id's can be shared over other communications channels if need be. Scanning QR codes is the easiest option, however.