Try to contact other hackerspaces (https://wiki.hackerspaces.org/Poland). Mostly they can help out with stuff which could be needed in the long run.
What equipment do you have? 3D Printers, Sewing machines etc? You could make masks. That would be needed and is good publicity.
Make a nice website and emphasize on the benefits of membership (own jabber/slack/whatever account, email, cheaper seminar fees and so on).
What is your focus? Do you a theme (like spaceship or just makerspace?). Concentrate on that. Just ask for help with different stuff. If people see they can contribute to something then i think they will come.
Well just my two cents.
This is a narrow definition of Hackers limited to specific opinions and values. That's ok if that's the mission of the organization but it's orthogonal to the idea of a hacker space which is about activity not belief an specifically, hacker spaces are about the activity of making and not socio-political goals directly.
Don't misunderstand me. I think free software is an important and powerful idea. But in terms of creation, being around other people who are creative is more important than being around people who match one of my particular opinions and I definitely don't prioritize the degree to which I value free software over the tools and process that facilitate creation. Good luck.
Next I’d get on social media and announce. Beyond that get on google maps, etc.