Just did not have time to Google, may be someone may have a quick answer. I was asked if it possible to share calender with someone outside our Exchange organization. Someone in a different organization using Exchange or gmail/yahoo.

Yes it is possible to share calendar but you need to change sharing policy for that. See this link: https://technet.microsoft.com/en-in/library/jj657494(v=exchg.150).aspx

Also check this one: https://support.office.com/en-us/article/Share-your-calendar-information-353ed2c1-3ec5-449d-8c73-6931a0adab88?CorrelationId=6e2421bf-b465-4012-9c78-ed4c9c582bbf&ui=en-US&rs=en-US&ad=US

Hi,

In exchange 2013, administrators can set up different levels of calendar access to allow businesses to collaborate with other businesses and to let users share their schedules with others. Business-to-business calendar sharing is set up by creating Organization Relationships. User-to-user calendar sharing is set up by applying Sharing Policies.

More information and detailed steps, refer to this document.

https://technet.microsoft.com/en-us/library/dd638083(v=exchg.150).aspx

Best Re