To give access to a single person. You have to create a group and add this person.
As a general rule, the use of groups greatly simplifies access management, since access rights are not modified, but rather the membership of a group.
If the EPFL Intranet option is not visible, this means that your site is not an inside site. Check that the syntax of your site address is similar to https://inside.epfl.ch/mysite.
Moreover, this option is only available (and therefore visible) for inside sites. It is not an option that can be activated on a standard site. You must request a new site if you already have a public site and want a site with restricted reading access