The only question I have is: why would they? I can see the reason why the job seekers would subscribe to your website but I don't see why employers would prefer it to other alternatives. Actually I can see one: to be able to identify which candidates have a criminal record and eliminate them from their pool.
Registering there as an employer can be an act of good will and generosity but that seems a bit awkward to make them pay for it, no?
BTW, you have to have a very solid security from the beginning. A leak of your database would not only threaten your business but negate most of the good you did.