Admins can only send an invite to an address on a domain that is switched on. 1 domain is on.
Demo accounts@example.org
People with an @example.org address can be invited.
Turning a domain off stops new invites to it. People who already have an account keep it, and invites already sent still work. To remove someone, deactivate them on Team. You can switch a domain back on here at any time.
180DC closes a 180dc.org address when someone leaves the branch. It cannot close a Gmail or other personal address, so deactivating that person on Team is what removes their access.
For an organisation whose people should be invitable, like a partner university. It is switched on as soon as it is added.