Also, the ham radio guys like groups.io, which is where yahoo groups migrated to. There are a lot of groups there on specific things, like Yeasu FT-857D transceiver. Groups.io has per-group file storage, which is handy for this.

Anyway on USENET, you can access it via google groups, but it's a bit non-obvious. Here is an example group:

https://groups.google.com/g/sci.math

But how can you find groups? You click on "all groups and messages" on the top, then enter a search term and hit enter. Then click "outside my org". When I do this for the term "france" it shows "4635 groups outside your organization". Open the list and look for the groups that have "." separators, for example "alt.france".

https://groups.google.com/g/alt.france

The ones without the dot separators are google groups- if you post, the message does not go outside of google. I don't know if you can create a USENET newsgroup through google groups- probably not.

It's kind of crap for discovery, but OK once you have a set of groups you use often.

Edit: actually a better way for discovery is a website like this: http://www.harley.com/usenet/master-list/index.html Once you have the name of the newsgroup, just append it to the groups.google.com/g/ (also the site provides a link).