I have Plex Pass membership and have Plex set up on my Xbox One and Windows 8.1 account.
No issues on either for me, but sent an invite for family member. They downloaded Plex on their Xbox One and created a free account. When the answered my invite for sharing, it is asking for them to purchase a Pass membership to view and play my libraries?
That is due to the fact that the Plex app for Xbox is still in beta and only available in the plexpass subscription. Someone with a free Plex account will not have access to it.
If you set up plex home and add the user that way, they will be able to use the xbox one application.
Someone put together a bit of a how to here: [How to use multi-user on the Xbox One](https://forums.plex.tv/topic/132749-how-to-use-multi-user-on-xbox-one/).