Time Capsule user account password change
This seems to be a recurring problem with TC devices. I have managed to setup users on a TC in the past, but never had others attempted to change the password. That may be a limitation to the user who created the accounts/setup the TC. Are they able to create their own accounts from their machines? If so, that may help you with the issue. 

In closing, I know most people get the TC with the hope of being able to backup and use it as a NAS, but there are a lot of limitations with it. Right now, there are a lot of other solutions that can accomplish Time Machine backups, and act as a full-fledged NAS. I am currently running such setup on a Netgear Nighthawk, and I love being able to backup my Mac AND Windows machines.

Further efforts: Start by digging Through this post. My second suggestion is to post directly in the Time Capsule Apple forums. You may want to follow this thread as well (unless it was posted by you :P)