I feel a bit dumb posting it :/ but I can't get this to work and nor can some users so ....

How do we open another users shared calendar?

Go to calendar,
click open a shared calendar,
choose from the GAL
it says "unable to display the folder. your profile is not configured to support this operation for this user"

Is there something else that needs doing to enable this behaviour? (and any document that will give me all this info so I don't have to ask silly questions next time ;-)