You can either add this one person to their own group and add that group to the space, or you can add them where it says "Individual Users" in the permissions section of the space you want them to see.
In order to make sure they don't have access to any other space, you have to make sure they don't belong to any group that has access to any other space.
First off, your configuration may impact external access. If you are using Confluence Cloud it shouldn't be an issue, but with Confluence server, depending on how access to the server is configured, people may need VPN creds to access.
So let's say that external people can connect to your instance. Just go to the space in question, select Permissions under Space tools in the lower left corner (with the Default theme) and add that account to the Individual Users section.
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