Edit within the browser via about:config or add them in the user.js file, easier to make it permanent and portable

> We use a WhatsApp group for co-ordination. The time and location of the meetup is announced there.

That's a hard no. You really, really should use Signal instead.

I suppose organizers choose the easiest way to contact people rather than relying on one of the least-used messaging apps that people won't install just to attend a meetup.

Edit: It doesn't even seem to be strictly required https://news.ycombinator.com/item?id=49181030

They could easily use both, if necessary.

Or link them and more via a Matrix server - but that would require a bit more work to do.