If you have a product but no users at all, I think you should go back to the step of idea validation. Make sure what you're building is something that people might use in the first place.

Multiple ways to do this.

Th easiest, imo, is to set up a simple landing page for your idea. Know before-hand who might be interested in your product, if it's just a handful of people, that works too. Just prepare for a lot of manual work (getting in touch with people and conversing about your product in different ways without making it seem like some sort of an ad), at least until your first 1000 users, more so for the first 500 (assuming your end-users are individuals and not businesses).