My setup: Plex on Synology. I have 5 accounts (one for me, and each of my family members). Everything works as configured.
A friend and I have shared our Plex libraries with each other. I can see his Plex libraries under my account but not under my kids accounts. Is this a limitation of sharing libraries (only the primary account can see them) or do I have something setup wrong?
It is a Plex limitation. Managed accounts cannot see libraries shared with the main account (you cannot re-share libraries shared with you).
For the kids to see the shared libraries, they would need full accounts at plex.tv (not managed accounts). Your friend would then share libraries with their accounts.