users will not go there.
most are 40+yo and have no intention playing around with the cloud. They just want their files. They do not expect their mail adress to be exploited to other users.
I answer maybe too late, but if someone is searching response, it can help anyway !
You can hide users by creating groups. After that, In the sharing settings in the admin section you can disable the autocompletion or limit it to groups a user is part of. (source)