Actually, what I think he wants is to be able to lock the server with a password so only those with the password can enter the server. In that case you need two cvars in you server.cfg file:
seta g_needpass "1"
seta g_password "yourpassword"
This prevents anyone without the password from joining. If you just want to reserve a few slots with a password and leave the rest open to the public, then create "private clients" with:
seta sv_PrivateClients "n"
seta sv_PrivatePassword "anotherpassword"
where "n" is the number of reserved slots. These are subtracted from your total number of slots, and the rest are open to the public. Of course, "yourpassword" and "anotherpassword" are replaced with passwords that you make up.