How do we associate client_id with any customer

We are an assessment vendor and we intend to start integrating our assessments with the Application Complete Integration. I am assuming we start off by fetching a client_id,client_secret combination after registering as a partner. I am also assuming this gives us a sandbox customer whose jobs, people, candidates we can acceess using the above client_id, client_Secret.

But eventually we want to be able to start accessing the data of a specific customer. How does that happen? Can the same client_id be shared with the iCIMS representative of that customer and he grants access?