code: public GroupMember getMemberFor(String commandId) {
          for (Iterator it = members.iterator(); it.hasNext();) {
              GroupMember member = (GroupMember)it.next();
              if (member.managesCommand(commandId)) {
                  return member;
              }
          }
          return null;
      }
docs: Returns the group member that manages the command with the given id, or null if none of the
  members in this expansion point manage a command with that id.
  
  @param commandId The id of the command whose managing member is to be returned.
  @return The group member that manages the command with the given id, or null.