If you already have “Plex friends” who do not have access, click on their profile picture in this list https://app.plex.tv/desktop/#!/friends
then on the vertical ellipsis ⋮ on top and then “Manage Library Access”
The version of the web app that’s included with your Plex server is older. It’s only intended to be used for setting up the server and signing it into your account.
Quite alot has recently been changed in the user/friends management, so the hosted web app is considerably more up to date.