Ask HN: What makes a good meet up?

A while back I undertook a project where I met up with people to teach them something and, in return, they taught me something (http://news.ycombinator.com/item?id=1883294).

The project received a great response and now I'd like to turn this into an event (or "meetup", if you will). Initially I'm envisioning a bunch of people gathering at a bar, everybody telling the group what they would be willing to teach, and then people pairing up to teach each other. Then depending on the response, I'd love for it to continue on a bi-monthly basis.

I've never put one of these things together before, so I'd appreciate any advice you have to offer. Some questions I have initially are:

How/where would I market this? What are some meetup formats that work? How much should I play with the format? How often should something like this convene? What is a good method for collecting feed back about the session?

Then there are obviously questions I don't know I should be asking. So really I'd be grateful for any advice :)

Thanks!