If I'm in that situation and I'm the developer then I would be focusing first with the security of the platform. You'll be getting information from your users and they should be confident that their data is secured and won't be used for the advantage of the platform's development.
Next to it, I'll attract employers first, if there are no employers, there will be no users that will use the platform, right? Like giving them some incentive as the platform is just starting out. Any incentive that can be par or more than the other established freelance marketplaces.