Thank you contacting Telerik support.
In order to export Sitefinity users, you can use the UsersManager and RoleManager.
You can look at the documentation for more information here:
You can make a service that export the users filtered by the desired criteria. You can include also DateCreated and DateModified so you can export only new and modified users in future exports.
You can then use the e-mail campaigns and import the generated csv and setup the mail sending to these users.
Let me know if I can assist you further.
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