At the moment, people can only have one email address -- however if you are the team manager, you can go to the Roster page and add the Parent again with another email address.
They could receive communications -- and if they wanted to sign in with that email address they could go to www.teamsideline.com's sign in page and click the "Not registered? Create a new account" link to register that email address. When they activated the email address they would see your team.