The grouping of nodes in a view page is not active as long as the "Theme Developer" Module is active. I deactivated it temporarily to check that the people page still works in principle, but I guess it will be needed again from time to time until all design issues are fixed.

Also, since the "Theme Developer" module will only affect admin accounts when activated, the site still looks ok when viewed anonymously/as "tuser", even while theme development info is active.