gistfile1.js

db.events.group(
  { 
    cond : { event : "foo" },    
    keyf : function(obj) { 
      return { month : obj.timestamp.getMonth()+1, day : obj.timestamp.getDate(), year : obj.timestamp.getYear()+1900 } 
    }, 
    reduce : function(obj,prev) { prev.count++; prev.users[obj.user_id] = 1; }, 
    initial : { count : 0, users : {} },
    finalize : function(prev) { 
      var count = 0;
      for (var user_id in prev.users) {
        count++;
      }
      prev.users = count;
    }
  }
)

// results
[{"month" : 10 , "day" : 20 , "year" : 2009 , "count" : 2 , "users" : 100},{"month" : 10 , "day" : 19 , "year" : 2009 , "count" : 2 , "users" : 100}]