Do you REQUIRE everyone to attend the retro?

One of the most interesting aspects of this question, based on my experience, is whether the Product Owner of the team "should" attend the Sprint Retrospective. On many occasions, I've heard it said, either by somebody on the team, or by a manager, that the PO should not attend the retrospective, and I must confess, I've always found that perspective to be potentially concerning. One of the anti-patterns I've observed, albeit rarely, that helps add clarity behind this perspective, is a situation where the PO has a direct report who is a member of the team, which tends to reduce the potential for true psychological safety. Leaving that rare situation aside, it's my preference that the Product Owner be present, and when asked, that's always been my position; I'm curious about the perspective that others have on PO attendance, in particular.

If they are on the team, then yes. 

Retrospectives are a critical element for the team's continuous improvement. Obviously you would want to educate the team on the value of the meeting and encourage them to attend. However if they resist citing other "urgent" work, then take a cue from Shu, Ha, and Ri. The student must first practice the kata as is. Following the rules. In time, as they master the concepts, they can decide to run experiments with attendance. However, at the beginning, the practice must be taught. The value must be clarified. The experience must happen with everyone from the team.