I don't wanna make two separate threads, so I'll ask both questions here.
1. When you lock a party, does it mean that nobody can join, even though people you've sent invitations to?
2. I logged in with my Facebook acocunt some time ago, but now every time I enter multiplayer, I'm automatically logged in with my Facebook name and picture, rather than my PSN username, and I have to log off manually every single time. Is there some way to disable this automatic log-in?
1) Nope. Other people can join your party through friends.
Until ND fixes it (aka "soon"), you have to tell your friends to lock the party as well. Make them leaders one by one and tell them to lock it.
2) You can disable it in your XMB FB settings.
@tap Because those friends sent invites? Or can they join friends without invite?
They don't need to send any invites. As long as the "join party" option is available, they can do that.
To make it more clear:
Player1 (party leader - party locked)
Player2 (party locked)
Player3 (party locked)
Player4 (party unlocked)
Player5 (party locked)
Player4's friends will be able to see the "join party" option.