I was looking over the invitation letter, and saw THAT listed under "You MUST BRING the following with you to the interview", but I am not sure what exactly "evidence of selective service registration" is. Also, if I filed N-400 on 5-year residence base, but was married to USC, do I have to bring marriage licence/divorce paper?

You can go to the Selective Service home page at https://www.sss.gov. Click on "check a registration." Input your data and it will give you registration date and number, as well as a link to request written verification.