Often times, employees want access to other mailboxes from without Outlook but want the mailbox to be kept separate from their own.
The best way I've found to do this is to go into Outlook options > Account Settings > Data Files > Settings > Advanced tab > add mailboxes here.
The one problem I run into is that when someone needs to access a former employees mailbox, that user has usually already been removed from the global address list. When the user is removed from the GAL, adding the mailbox to outlook always fails because it can't find the mailbox. Does the user *have* to be in the GAL for this to work?