install fonts in font book so they are available to all users?

Check Font Book Preferences to see the Default Install Location. Mine was set to User.

When you choose Computer, they're supposed to be placed in the main /Library/Fonts/ folder. In that location, all users can access the same fonts. User means the font(s) gets copied to the Fonts folder in your user account, which is why no other accounts can see them.

With Font Book, the best way to install fonts is to never let Font Book copy the fonts into a Fonts folder. Put the fonts anywhere you want outside of any of the Fonts folders on the drive. Make a separate folder for them on any drive you want that all users always have access to.

In Font Book, create a new Library Set. Navigate to where the fonts are and add them. The difference between that and the default font set is that the fonts are activated from wherever they reside. No need to have them copied to any specific folder.

I too had installed some fonts using the Font Book application from my admin account. After installation they were only available in that account. In addition, the Font Book sidebar had listings for "Computer" and "User" and the fonts were only listed in the "User" collection. Following Eric's information I changed the default install location in Font Book to "Computer". I then tried installing the fonts again using Font Book and got a warning error for each saying it was a duplicate. I figured I'd deal with duplicates later so I continued with the install. Nothing changed. Using Font Book I then removed the fonts in question and reinstalled them. This time the install process triggered a Finder verification pop up that required my admin account password. The "Computer" and "User" collections disappeared from the the Font Book sidebar and my fonts are finally available system wide. I surmise that Font Book won't install duplicate fonts so the only way to make fonts accidentally installed in a user account available system wide is to remove and reinstall with the correct default location setting.

While this thread is a few years old now it's one of the top hits on Google when researching this problem. Kurt's advice concerning manual font file placement is accurate but as Apple makes it increasingly hard for novice users to find actual files, especially ones located outside a user account, I thought my experience useful in that it shows it can be done using only the Font Book application.