General :: Filter Contacts By Group With Contacts Contract

I'm developing a big application and a part of it is about contacts. I want to, first, get all the groups of contacts. And when the users selects one, show all the contacts of the group.

I've seen other applications and widgets (like LauncherPro widget or HTC Sense Widget) than can do it, but when I try, I don't get same results.

My code is:

1.- To get an example group (this is just for testing, in real application user selects):

Code:
long idGroup = 0;
String[] GROUP_PROJECTION = new String[] {
ContactsContract.Groups._ID, ContactsContract.Groups.TITLE };
Cursor c = mcp.getContext().getContentResolver().query(ContactsContract.Groups.CONTENT_URI, GROUP_PROJECTION, null, null, ContactsContract.Groups.TITLE + " ASC");

[Code]...

2.- To filter Users by group??