Problem with basic functionality in Swarm

I have to be doing something wrong, because the basic functionality is just not working.

I have a test which I’ve reduced down to this:

Swarm.join(:foo, self())
Process.sleep(1000)
assert [self()] == Swarm.members(:foo)

But all I get back is a blank list. This happens in the shell too, no matter how long I wait.

I’m running this version:

  "swarm": {:hex, :swarm, "3.4.0", "64f8b30055d74640d2186c66354b33b999438692a91be275bb89cdc7e401f448", [:mix], [{:gen_state_machine, "~> 2.0", [hex: :gen_state_machine, repo: "hexpm", optional: false]}, {:libring, "~> 1.0", [hex: :libring, repo: "hexpm", optional: false]}], "hexpm"},

On the latest Ubuntu release, running in a VMware player VM, if it matters.