Class ContactGroup

// The code below creates a new contact and adds it to the "Work Friends"
// contact group
const contact = ContactsApp.createContact(
    'John',
    'Doe',
    'john.doe@example.com',
);
const group = ContactsApp.getContactGroup('Work Friends');
group.addContact(contact);

// The code below retrieves a contact group named "Work Friends" and deletes it
const group = ContactsApp.getContactGroup('Work Friends');
group.deleteGroup();

// The code below retrieves all the contacts in the group named "Work Friends"
const group = ContactsApp.getContactGroup('Work Friends');
const contacts = group.getContacts();

// The code below retrieves a contact group named "Work Friends" and gets its id
const group = ContactsApp.getContactGroup('Work Friends');
const id = group.getId();

// The code below creates a new contact group and then retrieves its name
const group = ContactsApp.createContactGroup('Work Friends');
const name = group.getName();

// The code below retrieves two contact groups, then logs whether or not
// each is a system group.
const myGroup = ContactsApp.getContactGroup('Work Friends');
const systemGroup = ContactsApp.getContactGroup('System Group: Coworkers');
Logger.log(myGroup.isSystemGroup());      // Returns false, if the group exists.
Logger.log(systemGroup.isSystemGroup());  // Returns true.

// The code below retrieves all the contacts named "John Doe' and removes them
// from the "Work Friends" contact group
const contacts = ContactsApp.getContactsByName('John Doe');
const group = ContactsApp.getContactGroup('Work Friends');
for (const i in contacts) {
  group.removeContact(contacts[i]);
}

// The code below retrieves the contact group named "Work Friends" and renames
// it to "Work Buddies"
const group = ContactsApp.getContactGroup('Work Friends');
group.setName('Work Buddies');