Hi - I am assuming that the Extreme will be the primary router attached to the cable modem - then consider the following:
In order to have a guest network you would have to enable it on the Extreme - you would give this network a name and password of its own.
You would then set up the Express as a wired or wireless extender and enable the guest network - note however if you use an ethernet cable, you would choose to create a wireless network with the same name and password as the Extreme in addition to enabling the guest network.
So to answer your question, the Express cannot be used by itself to produce and independent guest network - even if you created a network with a different name, if would still be on the same subnet as the Extreme which would defeat your purpose.
I am referring to using the Express to extend the network from an Extreme LAN port via an ethernet cable - the Extreme is connected to the modem via an ethernet cable from the WAN port - the benefit of using the Express is that you would be extending both the Main network and the Guest network and you would have wider coverage on both - see this article for reference: