Currently you can assign ArcGIS Pro access to named users using ArcGIS Portal. The problem is when a user goes off the company network signing into ArcGIS Pro fails because even though ArcGIS Portal is accessible to them ArcGIS Pro for some reason tries to contact the License Manager directly to get the license.
When configured with named user licenses for ArcGIS Pro, ArcGIS Portal should be the one destination that ArcGIS Pro talks to in order authorize with ArcGIS License Manager. ArcGIS Portal should check to see if the user is valid, then if they are assigned access to ArcGIS Pro, and then be the middle man between the ArcGIS Pro client and ArcGIS License Manager.
ArcGIS Online has been able to do this for how many years now?