It sounds like they don't have SecureTokens tied to their accounts. You can check with the sysadminctl command.
sysadminctl interactive -secureTokenStatus TheirUsername
That will prompt you for your password, after you authenticate it will return something like:
sysadminctl[23116:2369928] Secure token is ENABLED for user Their Username
Or DISABLED if they don't have a secure token.
If you need to grant them a secure token you use a similar command...
sysadminctl interactive -secureTokenOn TheirUsername -password -
That will prompt you for an admin password, be sure to use one that is already available at the FileVault unlock screen. Once you authenticate it will prompt you for their password in Terminal, have them enter it.
If it works you'll get something similar to "sysadminctl[23188:2372666] - Done!"
This is all new to High Sierra and has changed a few times since it's initial release. Hope that helps!
@The_Lapin Thanks for the info! I am meeting with one of the users tomorrow. I'll try this out.
If you do a search on Rich Trouton's website here....
https://derflounder.wordpress.com
...he has several posts about using FileVault. It's been very helpful for me.