I have an iPhone 4s with ios 6.1. I was sent an invite to join someone's photo stream. When I open the email (using gmail app) on my phone and click on "join this photo stream" it goes to a default page that lists what the system requirements are for using photo stream, saying to sign into iCloud and then open the invitation email and click on the "join this photo stream" button. I have icloud set up on my phone and both my photo stream and shared photo stream are turned on but it still won't let me access the photo stream. In case it matters, I also use an older mac (running 10.5.8). But I am not trying to open the invite on my computer so I don't know why that would matter.