In this case I'm not sure that's a bad idea.

- This is, or should be, a very motivated audience.

- The point of the site is as much advertising yourself as seeking a cofounder as searching the current offers.

- The data you enter when you register is pretty much the search criteria for cofounders.

People like to see what they're getting into before they give anything away, though. This applies as much to motivated startup founders as casual web browsers.