Case owners can assign both individual users and groups of users to their cases as collaborators.
When you assign a group to a case, members of the group do not appear in the Collaborators panel as a group of users, but rather as individual users. Furthermore, only the individual users who are in the group at the time it is assigned are added to the case. Users you subsequently add to the group are only added to the case if you assign them to it individually or if you add the group to the case again.
After you add a group, you cannot remove it except by removing its individual members. If you add a group to a case for a second time, its new members, as well as any individual members in the group who were previously removed from the case, are assigned/reassigned to the case. Duplicate assignments are ignored: if you assign a user to a case and then assign his or her group to the same case, he or she will not have double entries in the Collaborators panel.