The original (and official) groff home doesn't anymore show how to get the latest source from CVS.
Change into your directory where all the packages you are working with are located and type
cvs -z3 -d:pserver:anonymous@cvs.savannah.gnu.org:/sources/groff co groff
This will create the new directory groff with the latest stuff.
Later on, to checkout the latest than, change into the groff directory and call cvs update:
cd groff cvs update
The source code of the currently released versions of groff is available at the GNU host (USA) [via FTP ftp://ftp.gnu.org/gnu/groff/].
--- Yeah, we got it