Adding an element to an array of a mongoDB property using Node.js

I have a mongoDB collection that is full of documents that look like that:

{
firstName,
lastName, 
birthdate, 
phone, 
email, 
hashedPassword,
groups
}

groups is as an array of documents that look like that:

{
   gid,
   score
}

Whenever a user is added to a group I want to add a new document to the 'groups' (and remove one when he gets out of a group).

How do I do this with Node.js?

Using mongoDB shell its easy cause I can use update with $push but as far as I know I cant use $push with node.js....

Thanks!!