Many of the people involved don't have smart-phone, and certainly couldn't go through the process of linking an app to a laptop/desktop machine. Sometimes it's tough getting them to use email "properly", which is why I have concerns over them using email for confidential conversations in the first place.

They have email, the have browsers, they are almost invariably using laptops/desktops and not phones, and they are technically inexperienced.

I don't think Signal would be an option, but I don't know it well, and I could be wrong, so I'd be happy to be corrected.

Thanks.

Wire is a option (https://wire.com/en/). It can be signed up to with a email adresse and does not need a phone. The web interface is pretty simple.

https://wickr.com/ could also be a option, its free for up 2 30 users but as it does more the interface is also more complicated.

Signal must be setup on a smartphone before using it on desktop. What you listed in your reply is important for people to know to properly answer your question. You should consider updating your question. Good luck! Nothing else comes to mind that is simple and desktop-only.

I've just added it to the question ... thanks.

To be honest, I don't think there is anything that's robust, non-tech friendly, easy to set-up, and reasonably secure. It doesn't need to be secure against state-level actors, just against people accidentally copying in someone unintentionally.