You probably can't afford me. :-)
The legal definition of a contract is far broader than a signed agreement. All that is required for a contract to form is offer, acceptance and consideration. Contracts can be formed many ways including verbally and by course of conduct as well as by a combination of many written documents.
If there are contracts that bind Steemit Inc then Justin is bound by them in his use of the ninjamined stake because Steemit Inc, not Justin Sun, owns it.