Hi marker2001,
When you go into the attendant you are using, at the end of the selector codes (depending on release) you will see an option for zero / time-out. Change that to the group you want (71 through 74). I have done this before and it works well. Be aware that there will be no mailbox to cover the group unless you loop a station port around to a CO port and include the station in the calling group. You could give the station "line ownership" in that case.
-Chris