Have you investigated using ArcGIS Hub premium with initiatives built in your AGOL and followers/collaborators in Hub? That's how our Org is beginning to work with outside entities on common projects.
Side benefit: Since Hub Premium is essentially another AGOL Org, the AGOL to AGOL or AGOL to ArcGIS Enterprise collaboration tools work too.
@ToddMetzler yes, Hub has been a great fit for a few of our large, more complex collaborations involving external participants, especially collaborations with a need to support open data services or requiring a web site presence. However, Hub has a very steep learning curve and its wealth of functionality has generally proven to be overkill for users interested in simple collaborations.
For the use cases I outlined above, our users are looking for simple ways to collaborate with external folks, simpler than Hub at least. Their main goal is typically limited to sharing content via ArcGIS Online with collaborators both within and outside of our organization, and for read-only or read-write collaboration. Regular and Shared Update groups have been a good match for such use cases, though with the potential pain point I touched on in in my post: how to easily involve external collaborators.
Furthermore, such users have typically already formed their "community", prior to needing to incorporate ArcGIS Online into it. They are already collaborating in Google or Microsoft Productivity apps, cloud storage (DropBox/Google Drive/Box/OneDrive), Canvas/Blackboard courses, etc.; all of which easily and intuitively support user-managed collaboration.
While GIS can be a key component in various steps of the research lifecycle, collaboration is what sites at the core of the propose, collect, process, analyze, share, preserve, and re-use cycle. And, ArcGIS Online's support for collaboration, while improving, still feels like it leaves a lot to be desired when it comes to fitting users' needs and expectations.