I can't tell you anything useful on this one. I am running Ubuntu 12.04, and the Python is 2.7.3.
Import grp works just fine on this one. This link is useful to describe grp:
grp – Unix Group Database - Python Module of the Week
With any reasonable make based build process, I would say fix your error (add grp) and continue where you left off.
Yocto seems to have it's own rules though, and I have never been able to restart a build. So start those 3000 steps over...
Make sure that you try Python from the command line when you get it sorted out, and verify "import grp" succeeds.
