Yeah I wanted this exact same thing - one big aggregate shared album. Like a family vaca. Everyone has iPhones and can add their pic to the one album.
Instead it's always the creator of the album sharing with whomever. Just like the Facebook integrating with contacts - good thought but no perfect implementation.
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Second thought, in classic apple fashion, they'll release an update allowing for multiple people being able to add to a shared photostream and tout it as an all new feature they just originally withheld.