This assembly is in the GAC: Microsoft.SqlServer.Management.RegisteredServers.dll
How can I add a reference to this assembly in Visual Studio?
I can view the file in c:\windows\assembly\
As the others said, most of the time you won't want to do that because it doesn't copy the assembly to your project and it won't deploy with your project. However, if you're like me, and trying to add a reference that all target machines have in their GAC but it's not a .NET Framework assembly:
I don't know if there's an easier way, but I haven't found it. I also frequently use step 1-3 to place .pdb files with their GAC assemblies to make sure they're not lost when I later need to use Remote Debugger.