The Pre-Conditions to enable External Sharing is that, External Sharing must be turned on for that Particular Site Collection. It can be turned on at the tenant level by the Office 365 Admin and then separately can turn on or off for every Site Collection. It is important to understand the below scenarios, when Share Option is clicked at the Site Level : When you are logged in as a User with Full Control Permissions Or you are present in the Owners Group, you can share your Site with both Internal Users as well as External Users; When such a User shares, he also will an option to select one out of 3 permissions levels: Owner, Edit or Read - so accordingly, the Owner user can assign permissions to any of these 3 permission categories. When you are logged in as a User with Contribute Permissions (Or you are present in the Members group), you can share your Site with both Internal Users as well as External Users). A Member user cannot select the Permis...