How do I add/create SharePoint 2010 Foundation users

First, please verify that you are on SharePoint 2010 foundation domain such as mysharepointonline.com.  This method only applies to SharePoint 2010 Foundation - not Enterprise level plans.

Go to Site Settings -> Site Permissions

Click Grant Permissions in the SharePoint 2010 Ribbon

Click the Create near the top of the form:

Enter your username and email to create the user and then grant permissions to the site.