Platform: Opera 9.64, Windows XP 64 bit SP2, 8 GB RAM.
No, not Google Groups:
If you want to access Usenet you'll have to see if your ISP runs their own newserver. Or, failing that, a public newgroup server to log into.
For that, you'll need to grab the info and go:
To bring up the Manage Accounts dialog.
New Account Wizard dialog will pop up.
Select Newsgroups and then click Next.
Follow the wizard and you're set.
However, even though you'll be hooked into Usenet, you still won't be able to use Google Groups via Opera.
A "regular" Google Group is indeed just a Google Group. But many of the groups that Google calls "Google Groups" are usenet news groups. These can be accessed using any interface that supports usenet news group access.
Well, the Wikipedia article on Google Groups that you linked to says it all: