A subset of the groups that are available at Google Groups are usenet groups. While Google does not provide NNTP access (the protocol used by usenet clients such as thunderbird to access groups) to these groups, they can be accessed through any other usenet provider (usenet groups are federated, in that a post to one usenet server gets replicated to all other servers).
These are the groups you can access this way: http://groups.google.com/groups/dir?sel=33604954
Any of Google's private groups are only available via the web interface or any apps that Google might produce to give access. They cannot be accessed with a standard nntp client like Thunderbird.