Click on Account Info and select the Collaboration tab.
Under the Workspace discoverability, click the allowed domain drop down.
Select the domain in the list and users from this domain will now be allowed to login to your workspace.
To remove a domain from the allowed list, click the cross option on the domain.
✍️ Domains in the list are generated from existing workspace members. If you want to add a domain to the list invite a user from that domain.