Authorize other accounts

Questions
1

How can I authorize another user to operate my contract?

2

This example adapted from here uses the theme of an address book.

template Contact
  with
    myself : Party
    myFriend : Party
    address : Text
    telephone : Text
  where
    signatory myself
    observer myFriend

    choice UpdateAddress
      : ContractId Contact
      with
        newAddress : Text
      controller myFriend
      do
        create this with
          address = newAddress

Notice that myself creates the contract, as the signatory. Also notice that myFriend can exercise a choice on the contract, as the controller on the UpdateAddress choice.

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