Some of my voters share an email account. Can BallotBox let them each vote, using the same email account?

No.  BallotBox requires each voter to have a unique email address.  We have allowed duplicate email addresses for voters in the past, and it caused the voters a lot of unnecessary confusion.  Really, almost everyone has his or her own email address by now, and if someone does not, there are many services (e.g., gmail.com and outlook.com) that allow free email accounts to be set up in just minutes.