Exporting

When exporting a build for testing, you must copy over the steam_appid.txt file from the root of your Unity project to the root of the build output. Without it, Steam will not be able to initialize in your build. If you forget to copy the steam_appid.txt and you are using the development user interface, it automatically create one for you if you try to launch the game and Steam initialization fails. Note that this is only for testing development builds, and when you actually upload to Steam, you do not include the steam_appid.txt file.

Testing Multiple Steam Players

As mentioned in the testing documentation, you cannot test multiple Steam users on the same computer, as Steam will not let you launch two applications at once. However, most multiplayer features can be tested on the same machine using simulated players. However, features such as matchmaking and friends features cannot be tested by these simulated players, and thus you will likely want to test them with multiple real Steam accounts. While there is no direct way to do this on one machine, we have a couple of recommendations. First, ensure you are using a form of version control such as GitHub for your project. Then, the next steps will depend on if you have a second computer, or if you don't and will need a virtual machine.

Second Computer

If you have a second computer, set up a Remote Desktop Protocol (RDP) connection from your main computer to this second computer. Then, install Steam, Unity, and your version control software onto the second computer. Sign into Steam on a separate account on this computer. Now, whenever you push changes to your version control, update the project on your second computer. You can now play in the editor from each, or play builds on each, and connect to each other.

Virtual Machine

If you don't have a second computer, you could create a Virtual Machine (VM) on your computer. Once created, install Steam, Unity, and your version control software on the VM. Sign into Steam on a separate account on this VM. Now, whenever you push changes to your version control, update the project on your VM. You can now play in the editor from each, or play builds on each, and connect to each other.