Some 1,900 invitations have been put in the mail.
Each invite was individually stamped and features the royal cypher in burnished gold.
Guests for the April 29 ceremony at Westminster Abby include family, friends and dignitaries.
Neither St. James' Palace nor Clarence House are releasing the names on the guest list.
Britain's Sunday Express newspaper reported that footballer David Beckham and his wife Victoria are on the royal guest list.
In a statement, William's office insisted it would not disclose details of individual guests.
One guest already ruled out is Sarah Ferguson, the Duchess of York, the ex-wife of William's uncle Prince Andrew. Her office said in a statement she "won't be attending and she was never expecting to attend," but confirmed she won't available for hire as a high-profile commentator.
The Associated Press contributed to this report.