In case there’s any confusion, if all you want to do is get Turris Puesco running on your local computer you can ignore any downloads from this thread that aren’t the GitHub.
You want the green “Clone or Download” button on the GitHub, then click “Download ZIP”. Unzip the zip file, make sure the contents of it are in their own folder, rename that folder to wgrpg, then copy that wgrpg folder to the htdocs folder inside of the folder you installed XAMPP to. If you did it right and XAMPP Apache is running, then localhost/wgrpg should return the home page of the site. Make sure you have an empty dbwgrpg database in your PHPMyAdmin, then import the dbwgrpg.sql from the GitHub into it. This is located inside the wgrpg folder you copied into htdocs, inside the SQL folder. Import that, if you did it right it will take a few minutes for it to load. Once it’s done, you should be ready to create an account and play.
As far as open source/having multiple contributors, like he said, it already is open source*, and the way Git and GitHub work mean it’s already possible for more contributors. Anybody can fork the code (and a few people already have) and make their own changes, it’s just a matter of if they want those changes to be merged back into Cryptic’s master branch, and if Cryptic wants to approve those changes.
*Strictly speaking it’s not open source, since there’s no license published with the code, but that’s a pretty common oversight that doesn’t seem to apply in this case.