Adding text to the email sent when registering

It’s in /wp-includes/pluggables.php in the function called wp_new_user_notification() on line 614. You’ll probably just want to change $plaintext_pass with whatever your password is in the line $message .= sprintf(__(‘Password: %s’), $plaintext_pass) . “\r\n”; on line 630.

From what your saying, it doesn’t sound like you want to change the password that wp generates a user. If you do, read above, otherwise, read this:

The person above me is right, that’s where the function you’re looking for is. But instead of changing the value of $plaintext_pass, I’m guessing you’re more looking to alter the email that is sent out to them, then below the lines:

$message  = sprintf(__('Username: %s'), $user_login) . "\r\n";
	$message .= sprintf(__('Password: %s'), $plaintext_pass) . "\r\n";
	$message .= get_option('siteurl') . "/wp-login.php\r\n";

Add:
$message .= __('Whatever your message is goes in here') . '\r\n';

Just to let you know that I have a plugin that performs this function(defining the registration email), which saves editing a core file(which should be avoided), you can find it here:
New User Email Setup