Under 'Preferences', you can enter a location on your profile, so you could put your zipcode there (I just did). The downside is that this isn't searchable, so it'd be hard to compile location-specific e-mail lists from that. Another option is to create a Wiki page with the usernames and zipcodes of anyone who'd like to disclose them -- that way, you'd just need to PM the relevant people to get e-mail addresses to start an e-mail group.
Earlier this month, Metus did a post asking for LW-ers locations. I thought it would even more useful to have this information in visual format, so I created a Google map. You can access it on the website below. Unfortunately, I am terrible at this post-writing interface, so I can't get the image of the map to load onto here. You'll have to click the link to view it.
Original Post: http://lesswrong.com/r/discussion/lw/8sm/where_do_you_live_meetup_planners_want_to_know/
(If you haven't filled out the poll yet, please do it! If more people submit their location info, I'll add them to the map.)
These are only for US locations that had a 5-digit zip-code, or a city name. I may or may not map the other countries. If someone else wants to volunteer, send me a message, and I'll add you as a collaborator to the map, so you can edit.