
People who say they work for these websites mean just that.
They work for that particular website which is most likely the mother company of some umbrella companies. These academic writing sites try to start up and maintain various writing "fronts" as websites for the clients to sign up with.
There is however, only one company in charge of all of these sites The writers don't even know half the time what the umbrella companies are until it starts showing up in the communications with the client via PM. In order to join the company, you can ask for website details of the mother company from the client assistance officers.
Ask if they have an opening for writers and usually, when the response is affirmative, they send you a link to the mother site where the application process takes place. The umbrella companies also have "jobs" links on the main site, which lead you back to the mother company for application purposes.
Some of them give an email address that you can send your resume to then, once your application has been reviewed, you will receive a link to take a test within the mother company site and from there, if you pass the test that is, you will be given the keys to the kingdom where you can finally create your writer profile and start "working" for them.