How can one get freelancers registered on my freelancer website?
You are going to need to build a funnel so you can convert cold traffic using paid clicks.
I would start with freelance registration so you can get a base of people.
Once you have that base, you can start attempting to get paid clicks by business owners, or maybe a PR blast that gets some traffic your way from businesses.
Marketplaces are not easy, because it is a chicken and egg situation, that usually requires a ton of cash to get the wheels turning.
The reason why only a few survive, and others die or merge, is because of this reason.
If I were you, I would go super niche. Think awfully specific with your freelancers.
Designers for specific businesses or tasks
Writers for specific purposes.
RPA Developers that only concentrate in a certain area of a project.
Super-duper expensive pros.
Super-duper cheap help.
If you try and do it all, like Upwork, you are going to get smoked IMO.
Hope that helps!