Directory API now supports guest users:

https://github.com/jay0lee/google-api-tracker/blob/master/admin-directory_v1.json#L3852

Add commands like:

gam create guestuser
gam delete guestuser

Etc.

Note that I think guest users have similar or identical attributes to regular users. Ideally we'd reuse all our existing code to handle these and treat guest more as an attribute than a whole different object type. That may not be possible in every case but I'd love to not duplicate efforts around things like rich profile (phone/address/etc) handling.