I'd imagine you put up posters at the university, coffee shop, or library with permission that involve a math problem, simple cipher, or so on with an invitatio

I'd imagine you put up posters at the university, coffee shop, or library with permission that involve a math problem, simple cipher, or so on with an invitation and/or point of contact. Relevant xkcd: https://xkcd.com/356/ https://xkcd.com/356/