DaltaChat (use its fork called ArcaneChat on mobile devices) is super straightforward to start using. You just download the app, enter your nickname - note: no phone number, no email, even no password! - and you're ready to go. Then you need an invite link to the group chat and that's it. The onboarding experience is in fact much simpler than on WhatsApp, Telegram or Signal.
Also as DeltaChat doesn't require a phone number or other confidentials, it is much more private than Signal. And it has stronger encryption than the latter (AFAIK Signal doesn't encrypt the message metadata).
I've managed to even get my gandma using DeltaChat (ArcaneChat) so I think it's the best choice for people who are not very into tech.