You use the types in that assembly in exactly the same way as you do types in any other assembly, including those that are part of the .NET Framework. In the properties of the reference, you can set the Copy to Output Directory property to true and then, when you build your project, that DLL will be copied to the output folder along with your DLL and you can deploy them both from there.
There are tools that can merge multiple .NET assemblies into one but is there really any need to do that? It might even not be possible, depending on how AutoCAD loads plug-ins. It might be that it needs one DLL per plug-in.