Is there a way that as an ArcGIS Online organization user/Admin, I can add external organizations' ArcGIS Online users to our organization's groups? We are working to collaborate on some story maps and apps between state agencies and we have run into all sorts of hints that this should be possible. When I try to add these user's accounts to our project group, the box to only allow me to search for users within my own organization is locked on, and I'm unable to check it. Since I couldn't search for them, we tried sharing the link to the group page so they can request membership, but I am unable to see any of their requests to join the group. Their accounts have been configured to allow them to join external organizational groups, but it seems as an Admin with my organization's settings, I'm unable to even search for them.
If the group was created with access and update capabilities (meaning all members can edit all items) then by design, membership is limited to the organization. This setting can only be changed when creating the group, but if you create a new one and do not check all members can update all items, then you should be able to search and add users from outside the organization!
Hope this helps!