If you have hundreds of members, why not ask each for a couple of dollars to cover slack costs? Or use a different platform. Or self host something else. Not to be overly dismissive, but my first impression is that you have a bunch of devs finding something interesting and wanting to follow along rather than a key cohort of people who will "disrupt covid-19." I don't see why Slack should pay for a bunch of people to use their platform. They are in the business of making money, and offer reduced costs for non-profits. If that doesn't work for you, use something else.
What is wrong with a subreddit for this, for example. Or a mailing list. Or just self host a chat app.
What is wrong with a subreddit for this, for example. Or a mailing list. Or just self host a chat app.