abTest.js

var groups = [
  {
    query: {group: "A", createdAt: {$gte: ISODate("2016-01-01 05:00:00.000Z")}}
  },
  {
    query: {group: "B", createdAt: {$gte: ISODate("2016-01-01 05:00:00.000Z")}}
  }
];

groups.map(function(group, index) {
  group.cursor = db.users.find(group.query);
  group.count = group.cursor.count();

  group.userIds = [];
  while (group.cursor.hasNext()) {
    group.userIds.push(group.cursor.next()._id);
  }

  group.interactions = db.interactions.find({
    userId: {$in: group.userIds}
  }).count();

  print([
    "Group " + index + ":",
    "Total users: " + group.count,
    "Total interactions: " + group.interactions,
    "Interactions/user: " + group.interactions/group.count
  ].join("\n"));
});