Sure, you can add or remove members of your own groups at any time.
FAQ Category: Contact-groups
Can I create my own mailing lists in Google Apps?
Yes. Just open your contacts picker and create a new contact group. the group then appears in your personal Contact Manager. To open the contacts picker: When composing an email message: Click the To: link. When creating a meeting invitation: In the event details window, click Choose from contacts under Add Guests.