SQL71501: User has an unresolved reference to Login in Visual Studio (2012/2013) Database Project
If you create application specific logins (which you should) then you are going to come across this error when trying to build your solution.
To correct this error, select include 'Non-Application-scoped' object types in the options (gear icon at top) when you do a schema compare (right-click the database project to find Schema Compare).
You can then just import the logins into your regular project, and the references are sorted.
Note: If you click on the Object Types tab and it closes the dialog (which it did for me), instead use the tab key until Application-scoped is highlighted, then press the down arrow to highlight Non-Application-scoped and press the space bar. Now you should be able to click OK and see the logins.