Answer:
Generally when you receive a wedding invitation there is a small card in an envelope placed inside and it should have the bride's address on it and a stamp. All you have to do is tick off if you are coming and if you are bringing a spouse or partner with you. If you did not receive such an invitation, but invited then you could phone the bride to let them know that you are coming and if you are bringing anyone. If for some reason you need to send a letter then you could say:
Dear __________(include both the first names of the bride and groom)
It is a great pleasure to be invited to celebrate your special occasion on (put date here and time) and that you (will or will not be attending) and if you are then mention that you will bringing your wife/husband/girlfriend/boyfriend.
Kind regards
(sign your name)