So this is what an apple tech told me today -
a device on the guest network of a Time Capsule cannot communicate with any other device. The guest network is only for internet access.
It cannot communicate with a device (printer) on the main network either.
To allow your friends to print, you either have to let them log in to your main network and use your printer that way or connect directly to your printer using hard connection (USB).
If someone knows of another way, I'd love to hear it too.
I know this thread is old and should probably have gone under Wireless >> Time Capsule instead of Mac OS, but a few minutes ago I printed from my guest network...
AirPort Utility (at least v5.6.1) has a checkbox to allow guest clients to communicate with each other. I checked it and was able to print after not being able to since I got it last March. Maybe the label should be more clear that it gives guests access to USB accessories as well, not just networked machines. This is a Time Capsule setting (I'm using a Windows laptop), not a Mac OS thing. Hope this helps.