Try to convince them to use Zulip? zulipchat.com offers free hosting both at a level resembling Slack's free tier and at a higher tier that's free for open sour

Try to convince them to use Zulip? zulipchat.com offers free hosting both at a level resembling Slack's free tier and at a higher tier that's free for open source projects, or you can host it yourself or have some else do that since the software itself is free and open source.