I had some time and looked it up, it's called "remote security ". This YouTube video gives an example of it at about the 7:20 mark
If you watch the whole video, I personally wouldn't setup permissions the way he did. I create individual user accounts, a single group account, and apply permissions accordingly. If not, you'd be vulnerable to people on your network being able to modify your files.
And I forgot to mention earlier, there are multiple ways of installing plex server as a jail within FreeNAS. The first is the plugin that you're using now. The other is through a jail you setup manually. It's a little more up front to learn, but IMO better in the long run. You have more control over it and can update it when you want. And the process is a lot faster.
There are s few examples out there. I think I got started with this one.
https://forums.plex.tv/index.php/topic/102532-how-to-install-plex-in-a-freenas-jail/