After I csuped to the latest CURRENT-Source and had a quick look at /usr/src/UPDATING I found this entry:

NGROUPS_MAX and NGROUPS have been increased from 16 to 1023
and 1024 respectively. As long as no more than 16 groups per
process are used, no changes should be visible. When more
than 16 groups are used, old binaries may fail if they call
getgroups() or getgrouplist() with statically sized storage.
Recompiling will work around this, but applications should be
modified to use dynamically allocated storage for group arrays
as POSIX.1-2008 does not cap an implementation's number of
supported groups at NGROUPS_MAX+1 as previous versions did.

NFS and portalfs mounts may also be affected as the list of
groups is truncated to 16. Users of NFS who use more than 16
groups, should take care that negative group permissions are not
used on the exported file systems as they will not be reliable
unless a GSSAPI based authentication method is used.

This is really great news!