Anyone can start a group, either by e-mail or on a forum.
You can fill a 20GB laptop hard drive in no time !
People in your area meet in a parking lot within a certian period of hours, normally about four or more, and turn on their laptops with 802.11 wireless cards connected and share a folder.
The standard "rule" is to use Windows shares, the basic windows folder sharing settings, and on Linux they use Samba (SMB) which is compatible with windows shares.
802.11 works within about 300 feet, can go up to 54 Mbs and most networks will pass packets to other nodes, extending the overall range.
File sharing while mobile is un-stoppable !
Some people meet at malls or other popular places such as food courts. Using a script or other software, sharing can be done unattended while you have dinner.
Start a group in your area, pass the word and happy sharing !

CarShare, tonight 8PM at barkleys parking lot.