Need step by step instrux on adding external user to O365 Group, please

I need step-by-step istructions on adding an external guest to an Office365 Group. These are the steps I've been using and the results I am getting:

1. I want to add aan external guest with a gmail.com account, so let's call the user MyName@gmail.com
2. Getting the MyName@gmail.com to use Email with Ease is not a problem. That works fine.
3. When I click on Read Group Files is when the problems begin. (continue below image)

   External Guests Public group with guests · 5 members   Welcome to External Guests. Use the group to share messages and files, and to coordinate group events. Email with ease Start a conversation with your new group. Read group files Access files shared in your new group.

     Upon clicking Read Group Files, I am taken to an O365 sign in page asking me to sign in with a MICROSOFT account with MyName@gmail.com. I tried to create an account at Outllook.com with the user MyName@gmail.com and of course it does not create the account. 
4. So here is my question: If my external user wants to use his/her gmail.com account to join/fully participate as an external guest, what are the instructions I need to give them to do so?

Thanks for your time. It is much appreciated.

Pat