On the new user signup page, there is a field "Steam". Upon entering text into that field, a tooltip appeared stating... The field "custom_field_Steam" does not exist. The exact wording may be off, but you should be able to reproduce it. I'm using the release version of Chrome if that helps.
See this thread. http://community.gaslampgames.com/threads/cannot-set-steam-account-in-contact-preferences.1757/