As always there is a wiki page to answer this question ... but unfortunately it's very much a "work in progress" and not a true resource yet (like say the Per-Level, ie. Solo Hunting Guide):
http://wiki.project1999.com/Group_Hunting_Spots
But of course, it's a wiki, and the only way it will get better is if people edit and improve it ...
(And unfortunately as more of a soloer than a grouper myself, I can only contribute so much.)