New user notification

I'm concerned that new user notification isn't working on my project. A number of people have never logged in, and when I've followed it up with them, they say they never received an invitation. I told them to check any spam filters etc, but they're still not getting them. With one person I've deleted him as a user three times in order to reinstate him and get an invitation sent out, and he's still never got it. I'm using exactly the same address at which I normally email him. If this was happening with only one person I'd think it was him, but I've had two or three say they never received an invitation.

Is there any way I can personally contact my users and give them a password that will enable them to get in? Can you give me a 'generic' password which they can then change when they do get in?