The customary way is to address it as "Mr. and Mrs. Smith & Family".
You address the envelope to the person and only put "X, Y and guest" on the invitation.
The focus of an anniversary celebration is on the couple, and the invitation should highlight this as well. An anniversary is a commemoration of a wedding day, just like a birthday is a commemoration of the day when someone was born. Just as birthday invitations address only the person who is having the birthday, anniversary invitations should address only the couple.
The person who is sending the letter or other item. If it concerns something like an invitation and the address is different for the party the return address is still the person who is sending it. The party address would be inside the invitation.
second person
Decide if you're going to attend or not, and let the person who sent the invitation know.
Wedding invitation envelopes that are inserted inside the mailing envelope are usually left blank. The outer envelope for the wedding invitation usually has the addressee and the return address of the person that will be contacted for all RSVP's.
If you put a stamp on the card and an address to send it off to, it will go to that address. Otherwise, the birthday card may just get thrown out by the mail person, or perhaps the mail person will think its for them.
Some families include such information in the person's obituary. But if there is no address listed, a safe thing to do is address it to the entire family, using the deceased person's address. Contact the funeral director for advice.
invite all the person's friends and family members
* When a person reaches 90 years old they consider having their family and friends around them to celebrate their birthday of higher significance and generally people of this age don't really need much as far as birthday gifts and prefers your attendance as a family member or a friend to help celebrate a very special day of yet another year or hopefully more years to come. It's the love, laughter and memories that make a birthday celebration successful rather than gifts.
An invitation letter invites someone to an event. The invitation letter provides the time and date of the event the person is invited to.
Address it as you would any other person, FORMALLY! Just because they are close with you, doesn't mean that they don't deserve the same respect that any other person invited to your wedding does. P.S Congrats, your getting married!