Have done 2 hours of GDPR training this morning.
A personal phone number is protected personal data, along
with your DOB, address, salary.
Absolutely anything that can be used to identify an individual would fall within the remit of GDPR.
I suppose there’s the off chance that the manager has conducted a Legitimate Interest Assessment beforehand, but i can’t see a way that would be reasonably justifiable when its the simplest thing in the world to just ask “mind being in this WhatsApp group” And it certainly doesnt seem like the OP was reasonably expecting it.
Seems like a dick move to me, through either not giving a shit, or not bothering to think about whether someone might not want it.
Id just leave the group and ask not to be added without consent again rather than kicking off tbh.