What is Slack?
Slack is a group communication platform built around chat channels. (You can join Code for America's Slack at slack.codeforamerica.org if you want to learn more!) Slack typically only offers one nonprofit plan per organization, but some brigades have had success applying for their own nonprofit plan. Your mileage may vary.
What can I get for free?
Slack offers a generous free tier to anyone that includes support for unlimited users with a fixed quota of storage space for messages and files. When you hit the limit, old messages and files are automatically deleted.