Networking at VirtualBox

Working with virtualbox is not as easy as i was imagine. I had to make a network for my friend last literate domonstration, but i cant connect one to another guest operating system. I tried a night, and no result. Because of this cause, my friend can not demonstrate his application. I have no problem with my application, but my friends application need my application to demonstrate. Hehehe, like dependency in linux package. And now? I still trying.