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'.
In our case, the user was creating the calendar by mistake under a different account. Recreated the calendar under his account and the button appeared.
Right Click on one of the Calendars > Create New Calendar > make sure to select Calendars that is under the user's account.