I have an app where I want to create each user who signs up on the platform as a contact on the ConstantContact. I am looking to create contact from the backend NodeJS, without OAuth flow. But I haven't found any document for the REST API to create a contact with just API key and Secret key. Please advise or point to the document that helps me address the problem.
Currently you must go through the oAuth flow. We are looking in to other options; however at this time those other options have not been released.
You haven't provided a great description of your app platform, but I'm guessing that those users will end up becoming contacts in the Constant Contact account, so no those people do not need a CTCT account and do not need to grant access.
Only the owner of the Constant Contact that will be adding the contacts needs to grant access.