Photos for OS X makes it easy to add people to shared Photo Streams whenever you'd like.
Whether you've created some new shared Photo Streams or want to add someone to an existing one you already manage, adding them with the Photos app on your Mac is pretty straight forward. They'll then receive an invitation to iCloud Photo Sharing they can join on any iPhone, iPad, or Mac.
Before continuing, remember that in order to invite someone to a shared Photo Stream, you must be the owner of that stream. You can not invite new members to a shared stream that you do not own.
How to add someone to a stream with iCloud Photo Sharing and Photos for OS X
- Launch the Photos app on your Mac running OS X Yosemite or newer.
- Click on the Shared tab in the top menu.
- Click on the back button to get to the main stream view if you aren't there already. The Activity section is the default view.
- Open the shared stream you'd like to add someone too.
- Click on the members button in the top right of the menu.
- Click inside the To field and type the name of the person you'd like to add. If they're in your address book, they should automatically come up. If they aren't, you can enter their email instead of their name.
- Hit Enter on your keyboard to save changes and send the invite.