Sometimes, a family group may include a staff member or a buyer in your fair. It's by far the easiest for everyone if different email addresses can be used for the different roles. The same email address can be used, but it's important to know that the email address has to exist in the exhibitor group (family) FIRST--before that email can be used for a staff or buyer account. For your first year's fair, you just need to make sure that you don't create a staff or buyer account until after that person's family has registered for the fair.
It gets more complicated in the second (and after) years, when the family has not yet registered for the fair, but their staff or buyer account may have been copied over when the new fair was created.
See Managing an Exhibitor Group with Staff and Buyer Accounts for help resolving family login problems.