Sitecore Access Control: Adding New Users

If you have ever run into situations where you added a new user but then the user is not able to log in to the UI, it is because the new user is not assigned the role "sitecore content users".  You can either assign that role manually or assign the role "sitecore client authoring" which also includes the "sitecore content users" role.  There are other roles that includes the content users role as well.