In order to search Google Groups, you need simply to be signed in with Google Account otherwise you may encounter no results or various errors like:
There was an error performing the search. Please try again later.
Once you are logged in, you are able to search it as usual Google Search pages:
And as regular Google Search, it supports all ...
If you go to the Trends page and enter an empty search query (just click the blue search button with no terms) you should be taken to the results page.
What this will show you is (I believe) the top searches (under "Related"). There is also a side-nav filter option to show you the results of the last 30 days among other options.
You can also check out the ...