Wherever you want!
My new group meets at a public library. I've had a couple other groups before that met at public or university libraries.
They make good meeting places because there are books all around. It's good to have an atmosphere of books when working on your own manuscripts. Also, most libraries have little rooms you can reserve just for little group meetings.
But, there are other places, too. Personal homes (great for including snacks--like fresh cookies). Parks (probably only in the summertime...depending on your location).
Other public buildings...
Online. Email groups. Chat groups. Skype. (my group right now meets in person once a month and does online critiquing once or twice a month).
The key to picking a place is making sure that it's accessible to everyone and that each member agrees on the place. It's important that each group member feels comfortable in whatever location you choose.
Where do you critique?