In case anyone wants to know a way to share a photo stream between two different iCloud users and their iPhones/iPads or what not..
Let's say there is a husband and wife each with an iMac and an iPhone (keep it simple).
Create three iCloud accounts
Husband - email@example.com
Wife - firstname.lastname@example.org
Family - email@example.com
On the computers and the iPhones, set up firstname.lastname@example.org as the iCloud account. This will enable sharing of photos (plus, anything she takes will appear on his iPhone and both computers, and vice versa). In addition, you can do something like set up a family calendar or reminders if you like. Turn email sharing off (as well as anything else you don't want shared such as reminders, calendar, contacts, etc).
Then, on the iPhone, under the mail,contact, calendar settings, add a new account. For the husband, it will be his iCloud account. He will enable to calendar, email, contacts, etc. It can also be set up on the his computer, although it looks like for some reason the calendar is having problems.
Then, the wife will do the same with her iPhone and her computer. Now they both have a iCloud email, contacts, calendar, reminder, etc that are unique and not shared, but they also have a shared photostream.
(It works better with iOS devices and not as well with the iMac. On the iOS devices, you can add an account and set up sharing of bookmarks and such - on the iMac you can only enable mail, contacts, calendar.)
I have been doing this for the past week and a half, and it works great.