Much of the answer depends on what kind of application you are distributing. If it's self-contained and is intended for private deployment (not going in the Global Assembly Cache), then you can just provide a link to the testing group on a file share to have them copy it locally and run it.
Of course, your application may not be so simple to install and may depend on the existence of registry values, environment variables, file locations, etc. If so, I'd write an installer for it. If it's simple, I'd use the Microsoft Installer (MSI) to generate an installer for it. If it has more complex requirements (depends on third-party components, requires proper licensing, complex file, registry or environment configurations), then I would seriously consider purchasing a tool to help with writing the installer. The one thing to keep in mind is that the deployment of the application is as much a part of the application as any other feature, so it needs to be tested as well. Make sure that whatever the testers are going to do is what a customer would do, and installing it is one of those pesky things users can get into trouble doing.
This was first published in April 2004