I couldn't find anything on your router about a guest zone. But maybe I messed it when I looked through the web page on it. If it is a true guest zone (often called captive portal) then you are safe. That is if it is doing segmentation with VLANS, your better units also display an acceptable use policy or link to it, to help give you some protection if they do something illegal (since most guest networks are usually left open). You probably can secure your guest with something easy that you tell your friends so it's not open to everyone, but still keeping your friends out of your main network.
I'm just not sure what method DLink is using with this router to make the two zones, so I can't say for certain, but if you can point me to some info on it, I'll take a look.