Calendar Sharing

In the Organization>>Sharing tab, you have to enable individual sharing in your Office 365 tenant.  If you are trying to share between Federated domains, you would enable Organization Sharing.  Conversely, if your clients use Office 365 they too would have to enable individual sharing or organization sharing.

365 side: https://urldefense.proofpoint.com/v2/url?u=https-3A__technet.microsoft.com_en-2Dus_library_jj916671-28v-3Dexchg.150-29.aspx&d=DwMFAg&c=Dajmaoch1pCn1JcAX8dXPIf8nvCo1VxetCfffRjuIvU&r=ueLVNM-P1C8aWjLulZrHiMpmF52Q7IFtEImCDb6uaT0&m=pHF6Nn9kfUNIU4l6WKBji2JEl8bLi2Sa69Lp5SmtEYE&s=MdE9_nfuxWh7w7PvmAVkSb03nRZG_SZAm6A7OcWxg_s&e=

On-Premise side: https://urldefense.proofpoint.com/v2/url?u=https-3A__technet.microsoft.com_en-2Dus_library_jj657451-28v-3Dexchg.150-29.aspx&d=DwMFAg&c=Dajmaoch1pCn1JcAX8dXPIf8nvCo1VxetCfffRjuIvU&r=ueLVNM-P1C8aWjLulZrHiMpmF52Q7IFtEImCDb6uaT0&m=pHF6Nn9kfUNIU4l6WKBji2JEl8bLi2Sa69Lp5SmtEYE&s=y5ljbR8l5s2yqjSUEJNjFc0JvUqbObHT5P5bM_6FOFc&e=

How to add additional domains to your organization relationships: https://urldefense.proofpoint.com/v2/url?u=https-3A__support.microsoft.com_en-2Dus_help_2667844_user-2Dcan-2Dt-2Dview-2Dfree-2Dbusy-2Dinformation-2Dfor-2Da-2Dremote-2Duser-2Din-2Da-2Dhybrid-2Ddeployment-2Dof-2Don-2Dpremises-2Dexchange-2Dserver-2Dand-2Dexchange-2Donline-2Din-2Doffice-2D365&d=DwMFAg&c=Dajmaoch1pCn1JcAX8dXPIf8nvCo1VxetCfffRjuIvU&r=ueLVNM-P1C8aWjLulZrHiMpmF52Q7IFtEImCDb6uaT0&m=pHF6Nn9kfUNIU4l6WKBji2JEl8bLi2Sa69Lp5SmtEYE&s=ogQdaZ9_CO6XnX9wOyH2A3jonLQUKKv_hb5u7z8aSnY&e=

And why it does not redirect through hybrid – or why you have to change your autodiscover: https://urldefense.proofpoint.com/v2/url?u=https-3A__blogs.technet.microsoft.com_exchange_2016_10_17_the-2Dhybrid-2Dmesh_&d=DwMFAg&c=Dajmaoch1pCn1JcAX8dXPIf8nvCo1VxetCfffRjuIvU&r=ueLVNM-P1C8aWjLulZrHiMpmF52Q7IFtEImCDb6uaT0&m=pHF6Nn9kfUNIU4l6WKBji2JEl8bLi2Sa69Lp5SmtEYE&s=bSBKcicSvpsr08BhIFzOztpjd9HMk_knNoMh4b6OpEM&e=

Hope this helps.

David