There is a bug in the source of shadow src/groupmod.c which renders it effectively useless when PAM is enabled where it performs a null-pointer check for the wrong variable. See the following mailinglist for more information: http://lists.alioth.debian.org/pipermail/pkg-shadow-devel/2009-November/007850.html Affected version appears to be 4.1.4.2 only (so far?).
Created attachment 216305 [details, diff] Proposed patch for inclusion in the ebuild
thanks, added the patch to 4.1.4.2-r2