You only need to have an account on the site if you want to upload a new model or edit an existing model you previously added to the Gauge 1 3D circle groups.io site that has been transferred here.
You DO NOT need an account to veiw or download any of the models from the site.
If you do want to upload or edit a model, then Adrian has written a guide which you can access here:
About OpenSCAD
OpenSCAD is software for creating solid 3D CAD models. It is free software and available for Linux/UNIX, Windows and Mac OS X. Unlike most free software for creating 3D models (such as Blender) it does not focus on the artistic aspects of 3D modelling but instead on the CAD aspects. Thus it might be the application you are looking for when you are planning to create 3D models of machine parts but pretty sure is not what you are looking for when you are more interested in creating computer-animated movies.