Add an Existing Project to a Solution
You can add one or more projects to an existing solution. A project can be associated with more than one solution.
To add an existing project to a solution
In Solution Explorer, select the solution.
On the File menu, point to Add, and click Existing Project.
In the Add Existing Project dialog box, locate the project you want to add, select the project file, and then click Open.
The project is added to the selected solution.
Before adding an existing project from shared directories in a multideveloper environment, you should attempt to ensure that your changes do not adversely affect or overwrite another developer's changes.