Hello @DaveK - assuming that you want to use your AGOL org as the host and your Portal as the guest, and that you want to the Portal guest to Send Content by reference to your AGOL org group, you have to set the share level of all the layers in the Portal workspace/group to 'Everyone', and then set the sharing level for your layers in the AGOL group as appropriate.
As long as you have disabled anonymous access to your Portal, this should not be an issue. This is what we do to send all of our data to our AGOL org for our Open Data site.
If you want the Portal guest to Receive Content from your AGOL org group, then it really doesn't matter. You would have to enable anonymous access to your portal in order to NOT get a login prompt when you open the 'received' content in your portals' web map. You don't want to do that.
Typically, you would set up your field collection layer in your portal as I described in the first paragraph, and then have your field staff collect using a field maps form configured from a portal web map. You could then see updates to the layer in agol depending on the sync frequency.
