client.js

Meteor.startup(function () {
  Deps.autorun(function () {
    var selected = Session.get("selected");
    if (selected) {
      var hiive = Hiives.findOne(selected);

      if (hiive) {
        Meteor.subscribe("messages", hiive._id);
        Meteor.subscribe("activeUsers", hiive._id);
      }
    }
  });
});

Template.attendance.activeUsers = function () {
  return activeUsers.find(); //activeUsers is undefined! Even though Session.get("selected") changed from null to something.
};

server.js

Meteor.publish('activeUsers', function(hiiveId) {
  Meteor.call('canAccessHiive', hiiveId);
  check(hiiveId, String);

  // Setup some filter to find the users your logged in user
  // cares about. It's unlikely that you want to publish the 
  // presences of _all_ the users in the system.
  var filter = {
    'state.hiive': hiiveId
  };

  console.log(Meteor.presences.find(filter, {fields: {state: true, userId: true}}));

  // ProTip: unless you need it, don't send lastSeen down as it'll make your 
  // templates constantly re-render (and use bandwidth)
  return Meteor.presences.find(filter, {fields: {state: true, userId: true}});
});