A shared Photo Stream lets you create and share an album of images with anyone you'd like. And as long as they're using an iPhone or iPad with iCloud Photo Sharing enabled, they'll be able to like, comment, and get notified about any new pictures you add. To get started, you just have to create your first shared Photo Stream...
How to create a shared Photo Stream with iCloud Photo Sharing
Launch the Photos app on your iPhone or iPad.
Tap on the Shared tab in the bottom navigation menu.
If you're in the Activity menu, tap on Sharing in the upper left corner to return to the main view. If you don't have any shared streams, you can skip this step.
Tap the plus sign in the upper left corner.
Give your new stream a name and tap on Next.
Add the people you'd like to add and then tap Create.
Tap on the stream you just created.
Tap the plus sign to add photos.
Select the photos you want to add and tap Done in the upper right corner.
Type what you'd like to say about your images in the popup window and tap Post.
That's all there is to it. You can add new photos or remove them at any time.
If you want, you can add more people to the shared stream or change your invitees' sharing preferences by tapping the shared stream in question, then tapping the People tab along the bottom of the screen. Here, you can remove or invite new people; add posting capabilities for your invitees (so that they can share images in the shared stream); create a public website for the images; enable or disable notifications; or delete the album outright.