Disk scan shows account Guest uses 74 gigabytes; Can I reclaim space? The DiskExpert Utility shows 74 GB of disk space allocated to user "Guest", yet Finder only shows less than one GB. 

Can I reclaim the space, or is DiskExpert "lying"?
Disk Expert may be giving you the spaced used by "Users" which will include your stuff as well as Guest Users. If this is the case then erasing stuff under "Guest" will not buy you much. When I run Disk Expert I get space used by "Users" and the space used by me. Did you get a space for your user account? You should have unless you are the guest user. I also got a space labeled as "Files of user: Guest - Size 13.4 GB" so you should have similar spaces from Disk Expert. That is you should have a space for User that is bigger than the space for your account and the space for the Guest account. You should also have a space for both Guest and your home directory. By space I mean a section of the mapping for Disk Utility. See image below.



If you are sure the 78 GB are from guest users you should contact them and see if they need any of the info on the Guest Users account. If not you can delete the files under the guest user account. Be careful not to erase other directories and make sure there is nothing under guest user that you want to keep. The Guest user account was intended for "guests" to use your computer. If you haven't allowed others to use your computer I would not delete anything until I knew what was there.