I've used Elster for a number of years now (first because I was required to, as I did some freelance translation work; now to get taxes back).
I'm afraid I've never submitted a tax declaration for an organisation, so I can't speak to that angle, sorry.
To your question - in principle, I would say "yes, it is that simple" :D. And typically, you don't need to fill 0's in everywhere either, you just leave the field blank :) . What makes it trouble / work is knowing which fields you do need to fill out. Eg in your case, is it really just the membership fees you need to enter? Otherwise, I'd argue its simple enough to fill in.
Oh final point / caveat I just noticed: I've in fact not been using the Online version of Elster ("Mein Elster"), which I think you're referring to. I've used "Elsterformular", which is a Windows based version, and I guess - depending on your preferences - might be more comfortable for you. It's pretty hidden on the Website nowadays, so here's a link: https://www.elster.de/elsterweb/infoseite/elsterformular
Addition pointed out by @cbeleites, which is a good tip: If you're not sure about something, you can call the tax offices, ie your local "Finanzamt" (you can find that here ). They're usually quite helpful!
Hope that helps (if not the original poster, than someone else)!