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:
Affected version appears to be 22.214.171.124 only (so far?).
Created attachment 216305 [details, diff]
Proposed patch for inclusion in the ebuild
thanks, added the patch to 126.96.36.199-r2