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The PR of the Google Groups pages are very high (9) & I wondered if anyone who's made posting there has benefited from this in terms of link popularity.
I suspect not - a quick bit of link checking in Google for some poster's websites doesn't list Google Groups pages.
Anyone benefited from postings in Google Groups?
But to answer your question you will not get PR from the groups, but they are a great place to promote your site.
Before hence the song, they were heavily used to spam, but now are heavily monitored, so if you do add a url it better be on topic.