If you are the immediate supervisor of the bride or bridegroom, you may be invited to join the ceremony. Accept it, it's a great honour. Please do not dress in BLACK colours (Dress and necktie). For Thais, the black colour is reserved for a sad event like a funeral. I have heard many Thais complained about foreigners wearing black dress to a wedding ceremony and it is a sign of bad luck to the bride and the bridegroom.