Dynamically add fields to Mongo

I have an application where I would like to have a user make custom named groups, Im having a hard time figuring out how to have them save properly to the database. At first I was going to have groups saved like,

 groups: [
    {family:{
        type: mongoose.Schema.Types.ObjectId,
        ref: "User"},
        name: String,
        email: String
    },
     {friends:{
        type: mongoose.Schema.Types.ObjectId,
        ref: "User",
         name: String,
         email: String
    }
    },
    {colleagues:{
        type: mongoose.Schema.Types.ObjectId,
        ref: "User",
        name: String,
        email: String
    }
    }],

Then I realized that people should be able to add their own groups like work college or whatever they like.

I am using Mongoose and their schemaModels, so how can something like this mesh with Mongoose?

My application is using Node, so would I just to do...

User.findByIdAndUpdate(id, req.body.groupname, 
  {$push:req.body.group}, function(err, group){
   blah
  }
}

I just thought I would ask before opening up a can of worms, Im unsure if Mongoose would push back, or if $push is the right thing to use.