When the privacy for an activity is set to Sharable, the host can invite their friends, and these friends can invite their own friends. But these outer friends are not permitted to invite anyone.
For example, Justin creates a Sharable #BBQ and invites his friends. His friends will see a blue Pass-it-on button on that event Page. This means his friends can invite their friends to the #BBQ. If you are not friends with Justin, but you get an invite from one of his friends, then you can go to the #BBQ. But you will not see a blue Pass-it-on button on that event page, because you are not permitted to invite anyone.