Monster Hunter Portable 3rd let hunters exchange Guild Cards with other players in the vicinity, and Monster Hunter 3G will allow for this as well. On the Nintendo 3DS, this is done using StreetPass. You’ll exchange information automatically with other hunters you come across.
Another very cool feature in Portable 3rd was that players you exchanged Guild Cards with would appear as NPCs within the game. The same holds true for 3G — hunters you’ve swapped cards with via StreetPass will appear at the harbour, and you’ll be able to able to take on additional quests from them by paying a free. The game calls them “Furatto Hunters,” which means they come and go.
Guild Cards are customizable to an extent. You can select a pose for your character, a message to other players, and a title. Various titles are unlocked as you progress through the game. Here’s an example of two poses:
Monster Hunter 3G is slated for release on December 10th in Japan. In additional to the G-Rank quests and the special quests mentioned above, Capcom will also release new quests via DLC.