Accept it, it's a great honour. 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. On the same night, there will be a wedding party. The number of invited guests is around 100 - 300.
People like to organise the function in the hotel since the facilities are easier to set up.
Guests go to register the name at reception area. Guest will sign the guest-book wishing good luck for the couple. Guests will present the gift at the registration area. It could be gift or gift cheque. If you present gift cheque, value varies according your position and the position of the bride and the bridegroom.