When inviting an unmarried adult woman, or if you’re not sure if she’s married, use Ms.
When addressing an invite to an unmarried couple, write the man’s name first, followed by the woman’s (Mr. John Smith and Ms. Jane Doe).
For young girls under the age of 18, use Miss, and have her name follow the name of her parents if also invited (Mr. and Mrs. John Smith, and Miss Janet).
Addressing Wedding Invitations