Q:

Add permission to all authenticated users

I have a Site Collection in our SharePoint online tenant. I wanted to give Read permission for everybody in our organization. so in Site Permissions I Clicked on Grant Permissions and typed NT AUTHORITY\authenticated users and clicked ok. However nobody has access to the site collection yet. I thought I have to add NT AUTHORITY\authenticated users to the members group but still nobody has access to the Site Collection. 

A:

You can provide permissions to following groups :

All Authenticated Users: All user accounts which can be recognized by SharePoint Online, both for internal and external.
All Users (Windows) : User accounts from Federated domains.
All Users (membership): All user accounts from Office 365 Online Services.

A:

Everyone was what worked for me
EDIT
Everyone is a default group that contains all authenticated and guest users that can connect to your network.

