On my iMac how do I add people to share a Photo Stream once it is in the iCloud?

What version of iPhoto do you have?

From the article you posted from

Manage subscribers to your shared photo streams

You can add and remove subscribers to your shared photo streams at any time.

WARNING: If you plan to remove a subscriber from a shared photo stream and you think that person might like to keep some photos in the shared stream, you need to alert the person to save the photos before you remove him or her from the subscription list. When you remove the subscriber, your shared stream along with all of its photos is immediately removed from the subscriber’s Apple devices.

HideTo add and remove subscribers to a shared photo stream:

1. In the Source list, select Photo Stream.
2. Select the shared photo stream whose subscribers you want to change, and click the Info button in the toolbar.
3. In the “Shared with” section of the Info pane, do one of the following:
   • To invite a new subscriber: Type the invitee’s email address.If the person you’re inviting doesn’t have an iCloud account, click the Turn On Public Website button.
   • To remove a subscriber: Select the subscriber’s email address, and press Delete.

If you invited a new person to view your shared photo stream, an email and a notification are sent, asking that person to subscribe to your shared photo stream. If you removed an existing subscriber from your shared photo stream, the shared stream is immediately removed from the subscriber’s Apple devices. If you want to reinvite a subscriber to view your shared photo stream, delete the person’s email address from the “Shared with” section, and add the email address again.

LN