Summary

By default, although an administrator can configure approved domains or different methods on how users get site access, there is no option that would allow invites to be sent to users from specific domains.

Use case

If an admin invites a Gmail account, the user will be added to default groups and have access to content, which is expected.

If an end-user invites this Gmail account as well, it will go through admin approval if the User invites option is unchecked Existing users can send invitations to anyone (see https://jira.atlassian.com/browse/ID-7692 for more details).

Suggestion

To prevent people from public domains from joining an instance, even though it goes through admin invite (or approval), this is a suggestion to add a configuration that allows specific domains to be blocked from being invited (for example, Blocked domains in the Site access configuration).