Dynamic Query Strings appended to Sitemap urls

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C#

ASP.NET

ASP.NET4

I have a running web application that I am trying to intigrate in another group of people to use. My main goal is to have the sitemap urls append a querystring to the end of all the urls based off if they are in a group of users or not. I can already figure out if they are in that group, I just need help appending dynamically the query string.

Example: "www.page.com"
---(In group A)--> www.page.com?group=A
---(In group B)--> www.page.com?group=B


I could do it so that on pageload it checks and then response.redirects, but that would be slower making each page load twice, plus I would like them to have the ability to go to the other people site if they choice to among other reasons.

How can I make the sitemap dynamically append query strings to the sitemap? Can this be done or is it just easier response.redirecting?

Thanks for your time!