If a married couple is invited to a wedding, it is appropriate to reply as the incoming envelope was addressed. For example, if the envelope was addressed to "Mr. and Mrs. John Doe," the reply card should state, "Mr. and Mrs. John Doe will (or are unable to) attend."
When an invitation does not include a response card, the guest has the option of a formal response written in the same wording as the invitation or a personal note written with just first names. Personal notes should be reserved for hosts that the guest knows well. It is the guest's duty to reply by the date requested by the couple. While a formal or informal reply is acceptable, the reply should match the degree of formality that the wedding and reception will be held under.