Are there rules for playing Descent with more than 5 players?

Chris Park has a variant here: http://www.christophermpark.com/descent.php

Thanks! This is just what I needed. I purchased a few new games for my D&D group for X-Mas and did not even look at the number of players.

I've found the game to work fine with two players working as the overlord.

You should take note that the base game doesn't even scale perfectly for the numbers of players it claims to support (things get noticably easier for the heroes the more of them there are). You could probably do a lot worse than using the variant rules suggested above, but you should be prepared for a good deal of uncertainty in overall game balance and a significantly different experience than playing with fewer players.