We built a tool that was instantly useful to them, and in exchange we instantly had 100+ users. We released an extremely "M" MVP and had real users from day one. Because they were using it free and we were building features in response to their direct feedback, they were very understanding about it being a rough prototype. Two years later, they voluntarily opted to generously backpay us for use of the tool (we didn't even ask them to).
If you can get real users from very early on, even if your tool is rough, do it! It will help you build what's really useful to people, and involving early users in the design process actively means they are motivated to use the tool early and help you make it work as well as possible as quickly as possible.