Did you verify your account? I know they send emails. Theoretically, though, you shouldn't need to have an account for all the different instances. You can subscribe to a Lemmy community from kbin, using your kbin account.
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@Kayzels I know I can, but I'm just curious why I can't log in to any of these sites all with the same behaviour. If they sent me an e-mail I would have clicked it. I signed up a few days ago though I don't think I got an e-mail and going to sh.itjust.works shows the e-mail field as optional, so I can't really see it requiring a verification.
Edit: Found the issue. The site must have obviously lost my information. Despite the account being created, and me having a saved username/password, I've just discovered that there is no account. Recreated it and it now works. Absolutely no feedback from the site at all in regards to this.