I would so join but I don't have a phone number yet as I have yet to buy a phone, and I can't afford much anyway so paying for a phone bill or whatever doesn't sound like a good idea to me

you can register free sms number service like:
textnow
etc

that's a thing?

you register free account on textnow give you free virtual nubmer for recive sms or phone call on textnow site
you use it number to register bluesky account, blusky send sms on textnow virtual number,
on textnow site you just see that sms arrived from bluesky, open message and copy verification code to bluesky registration page
and done you have blue sky account not disclose actual phone number

Oh, neat! Interesting how they haven't gotten rid of the invites listing yet. XD