To see all the hidden nodes in a network type, in the Houdini textport type:
ophide Sop
This gives you a list of all the hidden SOP type nodes.
To unhide the Group SOP, just type
opunhide Sop group
Be warned that the main reason the Group SOP and Copy SOP have been hidden is that they are not going to work in a compiled fashion moving beyond H16.
As for splitting up the functionality of both Copy and Group SOPs, this was to get away from the funky way in which the Copy and Group SOPs worked.
Personally, I never really liked having the first tab group enabled always on creating a group when all you wanted to do was promote a group in the Combine tab. If you forgot to unset the Number > Enable option in the Group tab, you added that new group set to whatever. As well chording multiple options in the Group SOP was rare. The only time I really liked having more than one option was to create a group in the Create tab and then create a complementary group in the Combine tab. And the fact that to get at functionality it was two tabs down in the SOP Parameter UI. Lots of click navigation.