Honestly? It's unclear. The rules here are soooo murky, and so very dependent on so many things that there's really no telling. Some girls are simply okay with it -- hell, they even like it when some guys do this. Note, that's some girls. It all depends on things like context, culture (of the place, company, country, etc.), situation, history, the persons involved, the social standing of the persons involved, the physical appearances of the person involved, etc. etc.

In this specific instance (a meetup event, about a tech-related subject) one would certainly think that the guy was in the wrong. But yet I've been to enough business parties that would even put this to question -- I've seen girls react positively to behavior I would describe as groping like "that guy should be arrested" groping (I've inquired girls who were recipients of this treatment, they seemed genuinely fine with it -- that it was just "playful fun", and "not a big deal"). So, I really think that grand-parent post has the best answer: if you feel you're not being treated right, just say it then and there, tell them "hey, you're not being cool right now", "hey, you're making me feel uncomfortable, please stop with this, and don't do it again", whatever, just say it. That's the only surefire way to put a stop to getting harassed.