I used to go to a Ruby meetup and it was pretty much just like this. There was a core group who went every time and then you'd get some real whackos showing up. Often who didn't even know what the group was about, just that there was something on tonight.

I'm not sure a product is even able to solve this. A product needs to maximise engagement, turn a profit, etc. But there isn't really any money to be made here. A local community isn't going to want to pay a fee to some 3rd party so they can arrange to meet up at the pub. The best solution here is just a IM group chat. Only problem is it isn't very discoverable to new members, but to some extent being hard to find is a feature itself.