There are 2 kinds of visitors:
 - One kind of the visitors want to work in your city, there is a Mail icon above their head. You can tap the mail icon to check their information, and you can accept them to be your citizen and assign them jobs in your city.
 - One kind of the visitors just touring in your city. You can tap them to check their information. If you want, you can spend 1 diamond to invite them to be your citizen.