It is quite hard to differentiate designers without looking through dozens of design contests almost at random because your designers say so little about themselves. There are fields available for the designers to tell about themselves, but they seem to be rarely used, making most designers look very secretive and somewhat unprofessional.

I think services such as LinkedIn, Facebook and lately Google+ have shown that you create a better and more people friendly community by making people share information about themselves.

You should encourage your designers to give more information about themselves either by making such information required, or motivating with reminders, status or other mechanism.

You should also make some kind of designer catalog or listing, so that it is much easier for contest holders to find designers to invite for their contests.