I´m trying to share a calendar using Outlook 2010 but the button is greyed out.
I can share it without problems if I use Outlook 2007
I´m running Outlook 2010 against a 2003 Exchange server.
How can I share it using 2010 version
You should check inheritance is checked under the security properties of the user (in AD).
Also check full permission given on that mailbox to 'Self'.