If you get invited to a password protected game then you shouldn't have to put the password. Its just logical, that if the inviter wants you there, he would obviously invite you.

Example: "Hey John, why dont you come over to my house to play?"
"Sure, coming over in 2 hours"
*2 hours later*
*knock knock*
"WHAT YOU DIDNT HAVE MY KEY!?!? @.@

See? It just doesnt make any sense...