I had created a Homegroup on a Windows 7 PC a number of years ago, without incident. I was networking two Window 7 machines.

My now Windows 10 PC was unable to join this Homegroup, or access the shares. It kept telling me that I did not have the necessary permission or network password, etc.

I removed my Windows 7 PC from the Homegroup.
I then created a new Homegroup on my Windows 10 PC with a different network password.
I then joined my Windows 7 PC to the new network.

I am no longer having any network problems between these two PC's. I even rebooted my Windows 10 machine to make sure.

I hope this works for those of you who are having similar issues.