Its part of the Managed Preferences templates, or at least it was in version 8.x. I can't check to see if its still there in version 9.x It showed up as "Disable guest Account" under the com.apple.MCX section, but could also be used I believe to enable one. Just set the value to "false" instead of "true". We use it to specifically disable the Guest account and it works for that, but have not tried it in regards to enabling one.

Yeah I can't seem to find it in v9!

Well then, here are the values you need. Not sure if you can just make your own. Its not using an array so hopefully will let you make a custom MCX setting for this if you can't locate it.

Domain:          /Library/Preferences/com.apple.MCX
Key:             DisableGuestAccount
Value:           false (or true)

The setting under 8.x makes it into a System Level Enforced setting, which makes sense of course.

Thanks!

For some reason though it seems to not want to enable the guest account. I only have the policy scoped to one machine

https://www.dropbox.com/s/nhf3n5andqc0cxc/Screenshot%202014-07-29%2014.02.17.png

Thanks!

https://www.dropbox.com/s/wvavvots4rctzf1/Screenshot%202014-07-29%2014.06.35.png

That didn't work for me using 9.3.2 JSS
My machines are running 10.8.5