In the App Builder, you can choose to automatically share app items with non-workspace members who are tagged in a member (formerly known as a "contact") field. This can be enabled in any member field that is set to tagged workspace members by checking the box that says "Allow sharing this item with non-workspace members...". Any non-workspace members who are tagged in this field will then be automatically be invited as guests to the item.
This can be great for meeting apps where you want to be able to invite people outside the company to a meeting, or perhaps for assigning a deliverable to an external client.
Please note: Once you turn this on for the app, everyone that can post to the app will be able to invite people this way and it will happen automatically.