Signup sheet
There should be a signup sheet at midway to collect email addresses to email about the upcoming class. Ask for name plus email, so that if their handwriting is bad you have a second change to figure out their email, and maybe the class year to help you confirm you got it right. You can seed the paper with a few fake entries to make it look less empty. At the end of the event, make sure the list goes to someone to get the addresses added to the tech-squares-interest mailing list. When trying to decipher the addresses, in addition to regular finger, you can finger with wildcards against mitdir.mit.edu, or use this page, which is very handy when you can read part of the address.