Does jellyfin let you set up different accounts for remote users so they can keep track of where they are in a tv series (and not give them admin functions?)
Plex
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Yes
Ok, ok that's solid.
Does it have "library sharing" so one wouldn't have to login/logout to browse between my own media on my server and media a friend is sharing with me from their server?
They're totally different servers. You have to log in to each, but you don't have to log out of one to log in to the other.
I currently have three up in my browser, each in a browser tab, observing that my siblings and friend who self-host are all into some campy shit. I love it.
Yup!
Deleted my Plex account as soon as I got this email, using the account link in said email, so hopefully they see the connection.
All I know is I've been told over and over and over that Plex is better than Jellyfin because of reasons, so this latest move won't have me changing my mind!