This is more of a data modeling and permissions question, more dependent on the database choice rather than the framework I'd say. Since Meteor only supports Mongo right now, I'll use Mongo terms (collections & documents)

Meteor gives you an accounts collection so you don't have to declare it yourself. But we do need to declare the Groups Collection

    Groups = new Mongo.Collection('groups');

Let's assume we want Users and Groups to have a many to many relation, so each User doc has an attribute `groups`. `groups` is an array of which each element is the unique id of a Group document. Similarly, Group documents have an attribute `members`, an array of which each element is a User document unique id.

There is a third Collection needed, documents.

    Documents = new Mongo.Collection('documents');

Each document in the Documents Collection has an `owner` field that is the unique id of a Group document.

To handle the security for inserting or updating the Documents Collection, you need to set 2 allow rules on the Collection to check if the document.owner is in the user.groups array.

    Documents.allow({
      insert: function(userId, doc) {
        // doc's owner must be one of logged in user's groups
        return Meteor.user().groups.indexOf(doc.owner) > -1;
      },
      update: function(userId, doc, field, modifier) {
        return Meteor.user().groups.indexOf(doc.owner) > -1;
      }
    }

As for reading the document, you do that when declaring your Publications. Easiest would be to publish all of them, but you could also pass in a groupId or array of groupIds if you wanted to have some restrictions.

    Meteor.publish("documents", function() {
      return Documents.find();
    };