[feanaro]

You can create your own rooms (which are like IRC channels) which can be invite-only.

The counterpart to Slack workspaces/"servers" would be Matrix communities. They allow you to group a bunch of rooms and users together for discoverability. The feature exists today and is usable but still not as polished as one would hope for, but I think work on this is coming up soon.

In particular, I think better community front pages (describing the community, supplying related URLs and such) and access control (such the ability to restrict joins to community rooms to community members without having to invite each user to the room separately) are things that will be worked on.