Can't share a calendar

Hi,

I am new to using Outlook for Mac (version 15.6). I use Zoho for my mail provider and have successfully setup my email account in Outlook. I am now wanting to share my calendar with a work colleague, however all the permissions are greyed out - see attached and I can't mind anyway of doing this. Are the permissions not available because its not an outlook account?

Any help would be much appreciated.

Thanks,

Darren