It seems to require an invite from the owner of the community or members. The owner either gives you their contact details or require you get an invite from one of the members directly.

This is frustrating for me, an iOS dev wanting to join the iosdevelopers slack, because it requires an invite, and I'm a solo dev. So, though I have apps with millions of downloads and a decent amount of iOS dev experience, I don't really have anyone I know in the community.

I clicked on a discussion and at the bottom was a register button which directed me to a new page with another link at the bottom to register with twitter. Seemed to work for me.