Mongoose 'reversed' population, i.e. populating a parent object based on the reference defined in child schema

If you want to get a ProjectGroup object, which contains all the projects under that group. You can use Populate Virtuals. (Mongoose version > 4.5.0)

create a virtual schema in your schema file.

ProjectGroupSchema.virtual('projects', {
  ref: 'Project', // The model to use
  localField: 'projectGroupId', // Your local field, like a `FOREIGN KEY` in RDS
  foreignField: 'group', // Your foreign field which `localField` linked to. Like `REFERENCES` in RDS
  // If `justOne` is true, 'members' will be a single doc as opposed to
  // an array. `justOne` is false by default.
  justOne: false
});

and query in following:

ProjectGroup.find().populate('projects').exec(function(error, results) {
  /* `results.projects` is now an array of instances of `Project` */
});

If you cannot see virtuals part, plese set { toJSON: { virtuals: true } } to your model.

var ProjectGroupSchema = new Schema({
    projectGroupId    : String,
    title             : String
}, { toJSON: { virtuals: true } });

You can achieve this by using aggregate function. First group projects by "projectGroup" and then populate result.

project.aggregate([
   {$group: {_id: "$group", projects: {$push: "$$ROOT"}}}
],
  function(err,results) {
    user.populate( results, { "path": "projects.subscribers" }, function(err,results) {
        if (err)
         console.log(err);
        res.send(results);
    });

});