at the Mises Wiki, a new page has been created with a list of Austrian groups all over the world. If one lives nearby and wants to meet other sympathizers in real life for a friendly discussion, feel free to check it out.
If you know of other such groups, please add them to the list.
Mises Wiki | Economic Resources and Books (search engine)
You can find a lot of the Mises and FEE speakers on Facebook, too.
While I don't know how many would specifically identify as 'Austrians' I doubt you are going to find any bigger collection of market anarchists than in New Hampshire. There are certainly some proper Austrians up here too, but most just know of the Austrian school and occasionally read an article from mises.org, more from LRC, will claim that Austrian economics > Keynesian economics, but wont be able to describe the business cycle or know anything about Austrian methodology, etc. But in terms of a social community of libertarian anarchists, Keene and Manchester both have multiple times a week meet ups of decent side crowds. And although not terribly big in gross numbers, Grafton, NH might have the highest per capita numbers of market anarchists of any town in the history of the world (somewhere between 1-2%)