Just found this neat little Twitter tool: http://www.blastfollow.com/

If you have hosted or attended an event that you enjoyed and want to instantly follow the participants, just enter the hash tag (#) created for the event and hit Get Users.

Once that list has appeared, you will be able to automatically add these people to your Twitter “Following” account by entering your user ID and password.

This a great way to keep in touch with folks after an event and continue to market to them.