If you are activating the users manually (and using MODX Revolution) check out the ActivationEmail plugin.
http://bobsguides.com/activationemail-plugin-tutorial.html. That will send email to the user.
I'm not sure I understand your question, since the admin would be activating the users and wouldn't need an email.
If you're using the Login package's Register snippet, Register, in combination with the ConfirmRegister snippet, will automatically send the user an email with a link to activate. When they follow that link, they become active, and they can be forwarded to a page where you can put any message you want -- a snippet on that page could email the admin, or again, you could modify the ActivationEmail plugin to do it.
You can register at
http://bobsguides.com to see the process from the user's perspective.
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PLEASE, PLEASE specify the version of MODX you are using.
MODX info for everyone:
http://bobsguides.com/modx.html