Can you only collaborate with items and web maps? I am trying to share a Web App I built in AGOL to my Portal and I can only see the web map, not the WAB app. If this is not supported yet--will it be?
Support for sharing Web Apps in collaboration has been added to ArcGIS Enterprise 10.6.1. However, you will only be able to share your Web App from the 10.6.1 Portal to AGOL and not the other way.