https://astrachat.com/ paid XMPP. It claims the US army, apart from a few other government entities around the world, use it. I find this funny because this implies someone integrated Grok into XMPP.

Also, the old name for XMPP is Jabber. That sounds less techy, you know.

I bet that if someone scraped together the bare minimum and found a catchy name it'd be the winner easily.

Snikket.org

Yeah but XMPP sounds so techy you know... /s

the trick is to not mention XMPP to the end users. if they dig deep in the app settings they can find it out, but otherwise all that end users need to know is just, download the app named conversations, and use this domain and username. the domain could likely be hidden from them by deep linking, and sending the link in a company email. then they just need to log in as anywhere else.