Under certain conditions (in our case, possible internet proxy/configuration issues) ArcGIS Pro takes up to two hours 'Initializing' on start-up and afterwards you must sign in using your ArcGIS Online account, subject to you being given licence authorisation by your ArcGIS Online administrator (assumes your organisation is licensed for Pro). https://pro.arcgis.com/en/pro-app/get-started/manage-licenses.htm
This online requirement can also cause half-hour 'hangs' while working with portals or licensing where the application idles while it (presumably) attempts to talk to ArcGIS Online.
Once you have managed to successfully authorise your installation of Pro online, you can tell it to check out a licence so you won't have to do the above again: https://pro.arcgis.com/en/pro-app/get-started/view-software-licenses.htm
Note: There is no other way of authorising Pro asides from the above, even though loading licence files is given as an option in the application (see pic below):
I understand you would like to know if it is possible to authorize ArcGIS Pro Beta 5 without logging into ArcGIS
Online. Unfortunately, there is currently no way to authorize Pro Beta 5 without signing in with your AGOL
credentials. -esri support
In my case, in addition to the above, my browser had an extremely restrictive set of internet security settings (no active scripting, etc) which meant that I received errors like "ArcGIS Pro: The specified portal could not be set as the Licensing Portal. Please make sure the URL is correct and the server is running." (The URL was default: http://www.arcgis.com)
Once overcome, the logon box (IE custom window?) that appeared had almost nothing in it (just a few words), so it was impossible to use Pro.
It's probably unrelated but after managing to get Pro to work, afterwards when I tried to install the optional Python package, it told me I was unable to do that as 'another program was already installing'. After killing off about 4-5 rogue msiexec.exe threads, it finally installed.
System: Windows 7 Enterprise 64-bit, no other Esri products installed.