I know of a server that has private clients allowed, but I have no clue how to log on as a private client. Di I put something in my config file, or do I put it somewhere else. Anybody know how I can do this?

You should have sv_privateClients set to the amount of private slots, while sv_privatePassword is the password to access the places on the server.

To fill one of the private slots:
If sv_privatePassword is goat..
You need to type /password goat before you connect to the server.
So you will access the private client place, instead of getting a 'Full Server' message, or a public slot. ;)

Where do I type this? In the server in the console? In the Join Server Menu in the console? Thanks for your help.

If you use the MP 'join server' option, there should be a selection of Password which you should type in, before connecting to the server. Otherwise you can bring down the console, and type /password (value) instead. I think you have to type the password before joining, since you won't access a private slot if you connect and then enter it.