Our web site sends registration information directly from my cofounder's email address. (We don't like private messaging systems.) The plan is to start really personal conversations with all the people who sign up, until that gets too overwhelming.
Just as a test, do this: Pick 10 random users who sign-up and start a co-founder conversation. Pick another 10 random users and send an email saying "A member has replied to your message" or "A message was sent to you from Michael". See which is more effective.
You are discounting a very important factor - authority. Your co-founder is an authority figure, he's selling a site. He belongs in another group as the signed-up user. Another user sending a message is a peer. He tells the user that there are other people there, and that they are interested in what he is doing.
There is a big social difference in the two. Don't do one and never test the other.
