Justin Cooper writes in an email to Clinton’s deputy chief of staff Huma Abedin, “Fyi [For your information] clintonemail.com is down due to an outage with our ISP. Our actual systems are up. If it looks to be long term ie past tomorrow I will reroute the mail. [You] or Oscar can tell HRC [Clinton].” (US Department of State, 6/20/2016)
This indicates that Cooper is helping Bryan Pagliano to manage Clinton’s private server (at the clintonemail.com domain) and has some technical ability if he can reroute the email. However, he is not a government employee and apparently has no security clearance. Instead, he is an aide to Bill Clinton and likely spends a lot of time at the Chappaqua, New York house where both Bill and the server are.