Kate, 30, is the February/March cover girl on the U.K.'s Scouting magazine after performing some private volunteer work with the scouts.
"Part of her role as a volunteer is to help out below the radar," says a source of Kate's work with the girl and boy scouts and their younger counterparts, the Beavers.
The scouts hope the duchess will help run a variety of activities relevant to her skills and interests, and she is likely to be popping into some groups' regular meetings near her home on Anglesey, North Wales, and elsewhere.
Inside the scouting magazine, adventurer-broadcaster Bear Grylls pays tribute to Kate joining as a volunteer. "It is how we change our society: many people doing a little bit," he says.
The scouts are one of five charities that Kate has decided to patronize in her royal role. She is set to start officially touring some of them next month.
She and husband William, 29, are due back from a vacation in Mustique soon. William heads to the Falkland Islands early next month.