When we create a photo stream in previous iOS versions you can Share Photo Stream with friends and let them allow to view and comment on your photos. But in iOS 7, Share Photo Stream functionality is enhanced and give privileges to subscribers to contribute photos to Shared Photo Stream of friends. So, you can follow instructions below to create Shared Photo Stream and give access to subscribers can post.
Allow subscribers to post in Photo Streams
Your device and photo streams subscribers must have install iOS 7 of later versions on their devices.
Tap on “Photos” app icon from the Springboard of your iPhone
Tap on “Shared” tab from the bottom line, and then select the Shared Photo Stream from the existing ones, to which you want to allow users to add photos or you create a new Shared Photo Stream there, if you don’t have it already
After getting into the Shared Photo Stream, tap on “People” button at the bottom of screen
Toggle the slider to ON state next to “Subscribers Can Post” and that’s it
Now, your friends can also add photos to your Shared Photo Stream on iOS 7 device.