So this is for users who are already existing in Sitefinity
No, these are users Subscribed for the Newsletters. These are the same users in your Newsletter module.
How would I be able to add users who are on my global address book groups
You can either import these users to the Newsletter module, or just supply them in the tolist in the code:
You can also use the .NET SMTP and Mail classes with predefined list. This was just an example with the Newsletters module.
the Telerik team