Opening another user’s mailbox

How do you get another user’s mailbox to open? All I want to do is share contacts and the calendar, which I can do, but now in the folder list view it shows the other user’s mailbox with a plus sign, but it won’t expand. It says unable to show folders. What’s up with that? I can go to file>open another user’s…>and then open their contacts and calendar, but it would be easier if I could just expand their mailbox and click on the calendar and/or contacts. I hope this scenario makes sense… Please help. Thanks.