No worries, there is help on the way.
You just got the "user is not a valid group" error, which is common. The original Warpportal DragonSaga Installer does not work on non English Windows versions. And this results in getting that particular error.
Use this fixed installer instead:
http://forums.warppo...-2#entry1804397
Just simply do what Yuri told already another user. Use his fixed installer and all will be fine. You find it in the linked thread. No worries, it is completely save, add free and virus free. Over 1000 different downloads so far and many people on here used it and keep using it to install DragonSaga.
Just download the file and start it. From this point it will do all by it's own, including download and installing (you just have to tell it the folder where you want it to be installed to). It's easy.
After downloading and installing you'll have a 100% original DragonSaga, because Yuri just fixed the original installer so it can be installed on every Windows system.
And it is pinned by the DragonSaga CM directly in the Dragon Saga International Section of the forum. (just scroll down to the bottom of the forum and you'll find the area).