We have installed a fresh install of Exchange 2013 on Server 2012, using Outlook 2010 clients.

When users within the same office attempt to share Calendars, they simply click Share and invite the user.

This user then receives the email and they accept.

They are then allowed to see each others calendars.  However, they cannot actually see any appointments and the calendars are blank.  Is this a permission issue?

Anyone experienced something similar and able to make some suggestions how to resolve?

Thanks all

This should help you out.. 

Turn off cached mode on the outlook client --> Restart Outlook

Once you see the appointments -> re-enable Cached mode

Dame is correct,..

If you want to do this for external, then create a sharing policy:
http://technet.microsoft.com/en-us/library/jj657494(v=exchg.150).aspx

Doh.... so annoying.
I should have guessed!

Thank you, this does indeed fix the issue.