I have some Gröger from Thomasdorf in my research but no Wilhelm. However, there are some Wilhelm Gröger in Thomas Pompe's site at http://www.people.freenet.de/sonja.htm/ghtindex.htm
I have Thomas's email address if you contact me direct at lhakel @ earthlink.net (remove spaces).
I visited Thomasdorf this summer. Have names of other researchers with good information.