I understand that it may be problematic sometimes but this was very smooth. I didn't even say anything.
A: what's your number for the whatsapp group Me: I don't have whatsapp because of facebook. B: ok, we have to use signal then A: ok
And that was it. Life can be very easy sometimes
At first from the title it seemed like they changed app to avoid you
I wouldn't be surprised if that's how it plans out.
A regular group chat and another signal one for when you specifically need to talk to OP.