The user visibilities (who can see who?) in Confluence using the Space Privacy plugin depends on several factors.
Context (global or extranet space)
Space Privacy settings (visibility configuration)
Extranet space assignement (is extranet user or not)
Extranet role (extranet user or extranet manager)
the action you want to execute (search, mention, etc.)
To make thing clearer, the following sections explain the visibilities step by step.
Assume you have a Confluence instance with three users "A", "B" and "C", as well as two extranet spaces "1" and "2". All users are normal extranet users (not User Manager). User "A" is assigned to extranet "1", user "B" is assigned to extranet "2". User "C" is assigned to both extranets "1" and "2". The visibility configuration ist set to "Global administrators, extranet space administrators and extranet user managers", so extranet users can only see other users which are assigned to the same extranet space.
Extranet User Visibilities
See user in a global context
Global context means, that the content is not bound to some kind of space. Global actions are for example: the People Directory, the search, a users' profile or other third party plugin actions.
As the table shows, User "A" can not see user "B" (and vice versa), as they are not assigned to the same extranet. Whereas User "C" can see both "A" and "B", as "C" is assigned to both extranets "1" and "2".
Mention users in an Extranet
User can be mentioned (with autocompletion) if they are assigned to the current extranet space. Even if a user can be seen in the global context, you might be unable to mention this particular user in the current extranet space.
As the table shows, user "A" can mention user "C" in the extranet space "1" (and vice versa). Whereas user "C" can not mention user "B" in extrant space "1", eventhough he can see this user in a global context. This happens, as user "B" is not assigned to extranet space "1".
Restricting and sharing a page in an extranet (users)
These actions follow the same rules as mentioning user (see section above).
Extranet Group Visibilities
Restricting and sharing a page in an extranet (groups)
Now we add three groups to our Confluence instance: "X", "Y" and "Z". All groups are assigned as normal extranet users (not User Manager). Group "X" is assigned to extranet "1", group "Y" is assigned to extranet "2". Group "Z" is assigned to both extranets "1" and "2".
Restricting and sharing a page is only possible for groups that are assigned to the current extranet space.
The groups created by the Space Privacy plugin, so called managed groups, can not be used for these actions!
As the table shows, a page in extranet space "1" can be shared with group "X".
Pages in extranet "1" and "2" can be restricted to group "Z".