Taken from: The WordPress MU Manual
Welcome Email
This is the email that your users receive once they have activated their new blog or username. For example, here is what the default welcome email for a new blog currently look like.
This email can be customized to provide more information about your business while also providing your users resources to help them use your site.
And here is how Edublogs has customized their welcome email:
Here is the text that was used to create the Welcome email:
This is where the text was added:
First Post
This is the hello post that appears on new blogs that are created on your site. For example, here is what the default hello post for a new blog currently look like.
This welcome post can be customized to provide more information about your business while also providing your users resources to help them use your site.
And here is how Edublogs has customized their hello post:
This is where the text is changed:
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July 23, 2009 
James Farmer on Google+
Great post! If you remove the text from the Welcome Box will that disable it from going out? I think I want to keep everything in your premium follow up plugin, or would you do both?
For everyone else I am referring to this plugin:
http://premium.wpmudev.org/project/automatic-follow-up-emails-for-new-users
Hmmm… I’m not sure actually, but it;s worth testing and letting us know how you go ;)
Glad you like the follow-up emails plugin, that was/is a pet project of mine.