One of the links in the original Io9 article seems to be a reasonable repository of evidence for Asimov's groping.

The blog links to images from the rare books collection at NIU Dekalb, which hosted an exhibition on Chicon III, an early science fiction convention that Asimov was at. This archive contains letters that joke about Asimov's behavior.

Specifically, Earl Kemp, the chair of Chicon III, joked that Asimov should host a session on "The Positive Power of Posterior Pinching". Why? "Based on your delightful wit, and frankly your reputation." The blog links to an image of Asimov's joking response which seems to affirm the reputation.

But maybe it's possible that Asimov's reputation was undeserved somehow. Frederick Pohl says of Asimov:

When I last saw Isaac in the days before World War II moved Isaac in one direction and me in another, I would not have said he was particularly successful with women. He had not yet met Gertrude and neither Isaac nor his family had ever made me believe that there was any other girl in his life. But by the latter ’60s, he had become a good deal more adventurous. On meeting an attractive woman — one who was not obviously the Most Significant Other of some male friend — he was inclined to touch her … not immediately on any Off Limits part of her anatomy but in a fairly fondling way. (When I called him on it once, he said, “It’s like the old saying. You get slapped a lot, but you get laid a lot, too.”)

This seems to be fairly consistent with the above reputation.