Part-time organizations like learning communities, standards bodies, advocacy groups, hobby groups, and alumni organizations often need different things out of chat than companies do. While there may be some core members spending many hours per week on the organization, much of the value of the community may come from a large number of members or potential members who have an unrelated job and thus may have less than an hour a week to spend on the organization. In such an organization, making it easy for someone to participate when they don't have time to read everything can be the difference between a robust, growing community and one that stagnates.
Some of Zulip's earliest users were part-time organizations, so we have given a lot of thought to the problems such groups face. Zulip's topic-based threading
Makes the catching-up experience fast and fun, even if a user has been away for a while. On Slack or email, wading through hundreds or thousands of unread messages is taxing at best.
Makes it easy to respond to conversations that started hours or days ago, so that users that drop by occasionally can contribute rather than just lurk.
Zulip's topic-based threading also allows for more thoughtful discussion, since more people are able to chime in on any given conversation. It also makes it easy to start new threads, so digressions don't take over a conversation.
If you haven't read why Zulip, read that first. If your organization is a technical group that will be sharing code, you may want to read Zulip for open source as well. Finally, one of our earliest communities, the Recurse Center, wrote an extended blog post about how they use Zulip, which has suggestions for conventions you might want to include as you build your community.
Two additional points of note:
Pay as you go: We host many pro-social groups for free, and non-commercial entities at a greatly reduced price. Even if you don't fall into any such bucket, Zulip only charges for users that have been active in the last two weeks. So feel free to invite anyone you'd like, even if you're not sure if they'll end up sticking around!
Smart digest emails (coming soon): Zulip's topic-based threading makes it easier for algorithms to guess which messages and conversations will be interesting to users that haven't checked in in a while. Occasional interesting digests sent to inactive users is a great way to bring users back into the group.