Each SSO provider is linked to your company's email domain. To activate automatic workspace join, verify that you own the domain: the Enterprise SSO card gives you a DNS TXT record to add at your domain, then checks it (DNS changes can take a few minutes to propagate).
Once verified, new signups with an email on your domain join your workspace automatically instead of landing in an empty one, whether they arrive through SSO or normal signup. A domain can only be claimed by one workspace, first come first served, and public email providers (gmail.com and friends) cannot be used.