Not sure what happened for you but we've seen a steady stream of self-signup members today so the machinery is working. When you went to http://clojurians.net to sign up, what happened? You should have been redirected to Slack itself, and you should see a page like this if you're not already signed into Clojurians:

Then you should be able to select Continue with Email and signup.

(but we have no control over Slack's UI there)

Yes I’ve clicked on “Continue with Google” and it said something along the lines of “there is no account on that email” Perhaps it’s because it’s the first channel I joined with this email? Anyhow, no biggie must have been something related to my account...

You can't use Google or Apple, only Continue with Email -- even if your email is Google or Apple.

It's an unfortunate quirk of the Slack signup machinery, I'm afraid.