start by having a solution with two projects.
In the solution exploder window right click the solution and choose add New or existing item.
For this how two we will add a new c# class
click add. Visual studio will create a new suborder under the solution called "Solution Items" Class1.cs will be in this folder.
now select one of your projects and right click select add existing item. Navigate to your solutions directory and select Class1.cs
click the drop down arrow next to the add button
select add as link
You will now have a shared link to a common solution file/class
repeat for each project you wish to add this file/class to.