In OS X Yosemite, FileVault, "Enable Users" is not available
The Enable Users…-button is only available in the Preferences Pane Security &amp; Privacy > FileVault if one or more users is/are not enabled to use FileVault.

You may test this yourself: open Terminal and enter:

sudo fdesetup list


to list all FileVault enabled users

Remove one user:

sudo fdesetup remove -user user_name


wait a few seconds and open System Preferences > Security &amp; Privacy > FileVault.





Quit System Preferences > Security &amp; Privacy > FileVault.

Add the user again:

sudo fdesetup add -usertoadd user_name


and enter some passwords/recovery keys.

Open System Preferences > Security &amp; Privacy > FileVault et voilà: the button is gone.