Here comes a short guide on how to setup a Virtual Guest AP. In build 23020 I added an option to isolate guest wifis from local network. This along with my fix for unbridged vifs allows to setup a guest wifi with only a few clicks. The following image shows the Wireless->Basic Setup page. In order to create a virtual access point, for your guests, click on Add in Virtual Interfaces section. Now fill out the details for this guest wifi. The labels in red give you a short explanation what these settings are for.
Once you are done go to Wireless->Wireless Security, choose your encryption scheme and password for this guest wifi. Next step is to enable DHCPD for the guest wifi. Go to Setup->Networking and add another dhcp server for the guest network as shown in the following image.