Controlling Logins

Is there a way to send an email to a moderator when a new user wishes to sign up for a message board? The user would sign up and an email would be sent to the moderator. The moderator would review the sign up information and okay or decline that user? Our company is setting up a message board for a large number of users, but only a specific group and the information on the message board will only be available to those that have signed up and have been approved by the moderator.