You can add the assembly to the application's /Bin folder.
Or you can add the assembly to the Global Assembly Cache.
Adding an Assembly to the Bin Folder
The ASP.NET Framework automatically checks this folder for any assemblies.
If the folder contains an assembly, the assembly is referenced automatically by the ASP.NET.
When you add an assembly to an ASP.NET application's Bin folder, the assembly is scoped to the application.
If you add an assembly to the Bin folder, then you can take advantage of XCopy deployment.