You are usually added by someone else who is already a member of with my teens school there is a Facebook group where the admin will take phone numbers of anyone interested.
My advice though is swerve it.
I was added to the parent Facebook and it was utter nightmare fuel.
Parents seem to forget that others may disagree with their opinion and as a result start to behave worse than their children.
Our headteacher absolutely hates it and the WhatsApp group.
I actually ditched mine because the admin was like the Queen Bee at school who all the rest desperately want to suck up to. Think Regina George and you're halfway there.
The school decided to change the PE kit rules in late December 2020. They had let the kids wear black bottoms and tracksuit tops for years as long as they wore the (frankly overpriced) logo PE polo. So of course most of them wore Adidas, Nike etc.
Overnight, they banned any and all logo stuff. They also went on some daft offensive over sweatshirts saying they had to be zip up.
A few parents, myself included, voiced how daft it was, what with Covid and money being tight. That perhaps they could've told everyone before September, not say within 2 days you must change something done for years.
For some reason, and despite my comment bring politer than most and one of about 30 others the admin and her crew decided to write some of the most horrific and abusive messages back to me and only me. Along the lines of "if you're too poor to do as you're told you should leave the school" and "your kids are probably the type that everyone else hates because they don't follow rules down to you". The final straw was one of the crew has a child in my DCs class and came out with "no wonder your DD is the weird gay kid who dresses like a boy".
That was that for me. Bless the headteacher, she did speak to those involved without my knowledge but there's very little they can do about adults. And the stupid rule was actually dropped.
I would steer clear and keep your child away from pupil WhatsApps as well. They are awful, neither of mine bother.