The first-ever user created was Kuldar Krabbi GrabCAD's first software engineer to work on the project.
I suspect Indrek Narusk was second, and I think I was user nr 4. In the beginning, we didn't have URLs with names, but only grabcad.com/users/1 and I recall I had an nr 4 in the url. We all signed up the same day. The first version took about 1.5 months to develop.