How do I use a outlook email group name as determining who someone is?

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How do I use a outlook email group name in C# as determining who someone is?
I have been tasked at using a username / email address in a C# program.
Here are the steps that are involved.

1. A user uses the program that is part of a web based system
2. somehow the code (html, I suppose) determines who is browsing the web pages.
3. that username or identity of sorts, is compared to see if it belongs to an email group ID.
4. then the page activates or deactivates some controls, such as radio buttons, based on whether or not it belongs to one or more email groups.
5. Please advise. Where can I go to find samples or examples of the steps to do this task?