Simple Authentication Code to Access SharePoint Site

The goal is to allow some external users that are not a part of the company to have access to a single SharePoint site using a password or Authentication code. Does anyone know of a method that is very simple for the end user? 

Example:

• User clicks on a link to either a custom web app or the SharePoint site, enters in the code which then "authenticates" and redirects to the SharePoint site homepage.

The current authentication code process built into SharePoint requires too many steps to get logged in for our end users. 

SharePoint Built-in Authentication Process

1. Sending the user an invitation
2. User accepts the invitation
3. user requests a code
4. Once the code is entered it takes them to the myapps.microsoft.com
5. From there they can either click on an app in myapps or go directly to the SharePoint site

Any suggestions would be greatly appreciated.