In this tutorial I will explain how you could fix the problem "Invalid Guid".
Just follow the steps and it will work

First of all you should check if you have an etkey in your wolfenstein etmain folder.
Just go to the following path:
Linux: ~/. etwolf / etmain /
Mac OS: ~/Library / Application Support / Wolfenstein ET / etmain
Windows: C:/Program Files:/Wolfenstein - Enemy territory:/etmain
These are the standard paths, if you installed it somewhere else, just go there.
IF YOU HAVE ALREADY AN ETKEY IN ETMAIN.
If you already have an etkey in your wolfenstein etmain folder, then you just need to enable your punkbuster.
You can enable your punkbuster when you're in the server by these simply commands:
/pb_cl_enable & then you just do /reconnect (All in console)
Then your problem would be fixed. If not, follow the next steps.
IF YOU DONT HAVE AN ETKEY IN ETMAIN
If you don't have an etkey, then you should go to this site: link
Click on "Get ETkey".
Then you should save your etkey in the path where wolfenstein is installed, your etmain folder:
Linux: ~/. etwolf / etmain /
Mac OS: ~/Library / Application Support / Wolfenstein ET / etmain
Windows: C:/Program Files:/Wolfenstein - Enemy territory:/etmain
You're done. You can connect to the server with a guid. If you still don't have a guid, then it's because your punkbuster is offline. Just do /pb_cl_enable & /reconnect. Then everything will be ok!
I hope it will help you guys.
-W!ck3D