From a quick glance, the differences I see are: * In mattermost, different "teams" (or "realms" or "namespaces", whatever) exist on the same server (same url), and a user account that logs in will only see the teams they are assigned to. A single user account can be assigned to multiple teams (they appear on the left, similar to how the slack desktop app shows multiple server connections).

* Zulip requires a different subdomain for each "realm", and it sounds like users have to log into each one separately. It is not clear if the same account is shared between organizations or a user must have separate accounts.

So it sounds like Zulip's approach is separate, isolated "organizations", like slack, just hosted on the same server. Where Mattermost's approach is more like having separate, but integrated teams/groups/namespaces that a single account can be part of.