First I question the advice of your friend why you would need sub domains, that seems like over complicating things. People sometimes do that if they want to use some different software for forums for example it is not easy to integrate so they setup a separate forums.yourdomain.com so they can install the forum software separate from their site. But if you are just using features built into mojoPortal there is no reason to do that that I can think of, it just makes things more complicated without any good reason. The beauty of having everything integrated like the features included in mojoportal is that you do not need to have such complications. It sounds like a solution in search of a problem rather than a problem in need of a solution.