From what I've read it's only one to one/many. It's not a group album. So if someone wanted to share photos with you they'd have to set up their own stream.
Can subscribers of a photo stream not also add photos? Only the owner can?
Does anyone know if it's possible to view/change the people who are allowed to see a shared photo stream?
Yeah, go into the Photos app, at the bottom you'll see
Albums - Photo Stream - Places
tabs, tap Photo Stream and in there will be your shared Streams, if you click on them it brings up your subscribers, you can add or delete them right there.