Best to ask on wordpress.org
Don’t think that default exists any more, if you enable ‘allow site admins to add users’ then you get that option on the site admin users page along with role selection.
If you find out differently post back
A plugin likely to be used a bit, still not sure if we need to have these automatic assignments any longer – as per the other thread I rambled on in over this issue that mercime read – Boone made the point that it was unnecessary to have roles foisted on users and filling the DB with cruft. I guess I still am slightly unsure of a system that has floating users that do not belong to a blog as such but simply to the ‘network’ and without a perceived role – but then again form knowledge of other CMS’s this isn’t an unusual paradigm.