I did /tell mreyeball hidemyguild
It hid them as it did for 10 minutes, then when I checked much later, it showed all members are hidden for the next -3302 seconds (Underflow).
It also revealed every single location of everyone in my guild if I were to click on their individual names, not logged into realmeye, for clarity.
This has been tested twice and confirmed to be a consistent issue.
This is intended to be a high priority bug report for the realmeye.com service.
(Please move this to a more appropriate section if possible @Moderators).