Hi @amygupta7!
Welcome to the community! 
So naturally, environment variables only support one actual value. However, what you can do is make this value in a json format, and then in a pre request script make that json into a native javascript array and do whatever you please with it.
However, given the way you are trying to use your site ID in the request, you generally cannot pass an array as a URL Parameter. What you are looking to do then is make multiple requests for different site IDs.
Here I would suggest using the pre request script to fire off multiple requests with differing site IDs, combine those values then into one variable as a response, including the last request.
Or, you can use collection runner to run multiple requests with different site IDs, albeit they won’t be combined.
I am making assumptions on what you can do, as generally thats how REST services operate. Given that I don’t know anything about your web service, there may be other ways to achieve what you are looking to do.
Hope this help!