First, you need to create the user from your Control Panel first
1.) Loggin into your Control Panel
2.) Click on "Hosted Organization" menu
3.) Select your Organization name / Domain name
4.) At the left pane, click on "Users"
5.) Create the new user
When the user has been created, go to your Sharepoint site ,
~ Loggin as administrator
~ At the top-left corner, click on Site Actions >> Site Permission
~ Select the Grant Permission menu
