“Invalid guid” error appears mostly when you have no etkey in your etmain folder. Since Evenbalance stopped supporting ET, the “invalid guid” errors appears frequently, because while installing the game, it doesn’t automatically generate the etkey.
So, do this:
Go to http://etkey.org/ and click on “Get and ETKEY” button and download one, then unpack and copy it into your installed ET/etmain/ folder.
Remember: the etkey file doesn’t have any extension.
However you are able to join a server without the etkey, but there is a big chance to get kicked right away for “Invalid guid”.