You only need to add another user in the list if you want to grant him a different permission level than the Default group has.
You will first notice that the default permissions are changed for the Default group, to Free/Busy time.
Unfortunately, connectivity problems happen and Exchange 2010 free/busy information doesn't always display as it should.
Here are four areas to check -- and two tools to consider -- when free/busy stops working.
The longer the refresh rate, the less up-to-date your calendar will be.