This, basically. We have a LOT of part-time contractors to coordinate with. And would ideally like to open it to our customers as well, with more limited permissions.
Our take is that the full price (which is similar to Slack's) is designed for when the users are full-time employees that you're paying a rich-world salary, and that in that situation it's very cheap.
Here's what we say on our pricing page: https://zulipchat.com/plans/
> Zulip Cloud Standard is free for open source projects. We also offer steep discounts (usually 85%-100%) to many non-profits, educational institutions, groups of friends, and in other scenarios where most of the users are not fulltime employees of the customer. Generally, only closed organizations that also pay their members' salaries pay full price.