as of my knowledge, if you assign a task to a group and if one of the member updated the form, the task is in claimed stage. So no other people can work on the form.
Or: In your case, you might want to send a same copy of form to the group members as multiple instances. In this case, even if one member is acted and updated on the form, the other members can still see thier instance with the updated data (because all instances are refering to the same form variable).
Try to read the queue sharing concept also. I am not sure about that concept.
Also, just as an FYI, the next version of LiveCycle will contain a new "Multi User Task" service, which will send the form to multiple users, or all users within a group.