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
I have a feeling B wanted to use Signal, but expected it to be difficult to make others shift. When OP gave the opportunity, B came in and swyped it right away,