I just installed a new server after having to replace my old computer. The new server seems to be working fine except for sharing my libraries. I go to the share menu, select the library, then it shows a list of my friends. I select the friend I want to share my new library with, but when I hit the Send button, nothing happens.
If you shared with the friend before, you don’t need to send them a new invitation.
Just go to https://app.plex.tv/desktop/#!/settings/manage-library-access and edit each of your friend’s access permission to point to the new server.
If you just created the libraries on the new server, you may have to stop and restart that server, in order for the new libraries to become visible on that page.