Your not talking to anyone else but the Bride and Groom. Make it a personal message to your best friend without having to explain anything to anybody else. Share the memories you had with them (a funny memory, quirk they have and how you two met.) and offer your love and support moving forward. Doesn't have to be elaborate and you don't have to make sense to everybody there. Just your friend. This is your time to show the love and support you have for your best friend. I hope thinking of it that way helps.