How to allow a dynamic amount of users certain database permissions?

I have spent the past month or so developing an application using Visual Studio 2015 Community Edition and SQL Server 2005 which is a Windows Forms application that accesses a database allowing the user to perform CRUD tasks.

My *question/problem* now is how do I allow for a user on a different machine but same local network, to sign into the application (generic username/password) and start interacting with the database/application without any user permission errors.

I understand how to create users and grant certain permissions to said users but I am wondering of a way to do this without having to create new credentials/permissions for each unique person that uses this application.