So if I have a Master account, and my friends have lower-tier accounts and are part of a campaign in which I have turned on content sharing... what content is shared?

Is it just character builder content so they can make their own characters using the content I supply, or can they access everything? Notably, can they access the adventure I have bought via the compendium?

As of now, they can access everything: builder + listings + compendium.

Granular sharing is something that is planned in the future.

OK, so essentially they're on the honor system for now. Good to know. Thank you.