Only ever worked in the public sector before but if you're working in a small not-for-profit with no other full time staff or union members how do you know what union to join?
You choose one that is interested in having you as a member.
The only union that is interested will be one that wants to break into your sector. But there may not be any that are.
Well before you spend money on it, what exactly do you expect a union can do for you under the circumstances?