The bash in Norfolk, England, marked the first time the couple, both 28, were seen at an official event since their engagement last month, though they attended a private lunch hosted by Queen Elizabeth at Buckingham Palace.
"Between now and the wedding there will be occasions when Catherine does shadow Prince William, to learn the ropes and see how things go," a palace source says.
Palace officials have been stressing that, while senior members of the family and the royal household will help Middleton settle into her inevitable role, William has been taking the lead.
"William's a great teacher, so he'll be up to help me," Middleton said at her first photocall.
And William said in the official TV interview that accompanied their engagement announcement that he wanted to "learn from lessons done in the past" to give his fiancée the "best chance to settle in and to see what happens on the other side."