Dear all, I had the same issue and everything I've tried failed. Then, I've noticed something weird: the streams I couldn't join were the ones my Mother was also in... My Mother has her own Apple ID, but it's true it was I who set up her iPad (starting it, first, with my own Apple ID)... Then I've noticed that though the stream Autor has used my ID, AFTER it was sent, the stream showed my mother's name and her ID...
So what I did was ask the stream's Autor to remove my Mother (and me) and then add another mail of mine to the stream. I've accepted it on my Mac and it reverted to my Apple ID on the autor's side. Then my Mother was added again, and it was fixed.
I still haven't found the root cause of why my Mother's iPad get's my invitations when here Apple ID is her own... But this has fixed my issue.
After a recent system update, iPhoto was not joining new photostreams from friends, merely displaying older ones. I had also tried clicking the link in the email and pressing Join.
I followed all the tips above, however in the end these quick steps solved it:
1) In iPhoto go to Preferences (not the System Preferences)
2) On the Photo Stream tab, untick Shared Photo Streams.
3) Watch all the older streams vanish.
4) Once they're gone, tick Shared Photo Streams. The old and new ones appear!
I just tried all of these fixes but nothing worked. I'm trying to share a photostream from Aperture but. The invitation message is sent but when I click on the link "join this photostream", I get the error message "An error occurred while joining this photo stream. Please try again later.
Any Ideas what else might fix this?