Once you create a shared Photo Stream, you can share images with anyone you'd like. When you create a stream you're asked who you'd like to share it with. However, if you decide later you want to share it with someone new, or remove someone, you can do that just as easily.
How to add and delete people from a shared Photo Stream in iOS
Launch the Photos app on your iPhone or iPad.
Tap the Shared tab along the bottom.
Tap on Sharing in the upper left corner to get to the main list of shared streams.
Tap on the name of the stream you'd like to share.
Tap on People at the bottom.
Tap on Invite People.
Search for the person you want to invite and then tap Add.
To delete an existing person, just tap on their name and you'll see a Delete button.
That's it! Once the person accepts your invite, they'll be able to see the images anyone adds to that particular shared Photo Stream. One note: If you remove someone, remember that if they've saved pictures from the stream to their device, those images will not be removed. However, they won't be able to see anything new, and the stream itself will disappear from their devices.