code: public void deleteGroupContact(final String groupId, final String contactId) throws NotFoundException, GeneralException, UnauthorizedException {
          if (groupId == null) {
              throw new IllegalArgumentException("Group ID must be specified.");
          }
          if (contactId == null) {
              throw new IllegalArgumentException("Contact ID must be specified.");
          }
  
          String id = String.format("%s%s/%s", groupId, CONTACTPATH, contactId);
          messageBirdService.deleteByID(GROUPPATH, id);
      }
docs: Removes a contact from group. You need to supply the IDs of the group
  and contact. Does not delete the contact.