When shared iCloud Photo Stream albums first launched on iOS, they were a one-way street. Only the person who created the album and invited others to view it could add photos to the stream.
During the iOS 7 announcement last June, it was briefly mentioned that shared iCloud Photo Streams would soon have the ability for both the creator and the subscribers to contribute to the same album.
Fast forward to the release of iOS 7 and the revamped iCloud Photo Stream service, which now also allows for video uploads, and it's not far-fetched to assume subscribers are automatically able to contribute photos after making the upgrade.
While digging around a shared Photo Stream the other day I found the reason my family members weren't able to add photos or videos to the album: the ability for subscribers to post needs to be explicitly enabled for each shared Photo Stream.
Luckily the process is really easy, and it only takes a few seconds and a few taps.
Start by launching the Photos app on your iOS device. Navigate to the album you'd like to grant subscribers posting power in, and at the bottom of the album you'll see a People tab. Select it.
On the next screen you'll find a list of the people who are currently part of the shared album, with the option to invite more people. Just below the list there's a "Subscribers Can Post" switch. Flipping the switch will send an alert (see below) to those who subscribe to the album saying they can now contribute their own content.
In my experience, the option for subscribers to contribute defaults to off for existing shared albums, but to on for albums created after upgrading to iOS 7. Something that I think it's important to note when creating a new shared album after upgrading.