Basic version: you can sign up for internally shared services with an email account ending @company.name, the account gets sent a link to verify and give access.
But some @company.name addresses -- commonly support@ -- route the email responses to a publicly accessible page, eg a bug report.
So you sign up to Slack with that support@company.name email, get the link for access to the company Slack from the support website, Bob's your Uncle!
