This was an interesting one. My superior came to me the other day saying that people
were unable to add her to a meeting from Outlook because all they could see were slashes. Basically, this person’s free\busy information was unavailable to everyone. She asked me for help. Unfortunately, I didn’t know how to fix it at the time. But after trying a few things with several different people, my IT Director figured it out 
For people unable to view the free\busy information (or for people that only see the greyed out information [slashes] in Outlook), simply remove the affected person’s name from the nickname and automatic completion cache in Outlook. The “nickname and automatic completion cache” is commonly known as the NK2 file in Outlook 2003 and Outlook 2007. But it is different in Outlook 2010.
Here is an example of what I am trying to say:
I am trying to add my colleague, John, to a meeting appointment in Outlook on my own computer.
Under Scheduling, I can’t see John’s free\busy information. I only see slashes.
So on my computer, I need to remove John’s name from my NK2 cache.
http://support.microsoft.com/kb/287623
Once the name of the person is removed from the outlook nickname cache, all you need to do is just attempt to re-add the person to the meeting. You should now be able to see the person’s free\busy info without any problems.
**I need to give a shout-out to my IT Director for finding the fix for this problem!
(Don’t forget to update your Office suite to the latest Service Pack!)