In the Steam Library, right click "Mount & Blade: Warband" and select "Properties"
In the window that opens, navigate to the "BETAS" tab
In the "BETAS" tab, where it reads "Enter beta access code to unlock private betas:", enter the code invasion1234 exactly as shown
Click "CHECK CODE" and upon confirmation that the beta has been successfully opted into, select "invasionbeta" from the dropdown menu on the same screen
Close the window and allow the game to update
Your game will now be activated and you can join a server by filtering those compatible with your module and running "Invasion" mode or host your own!