From one silver to another, I think it's just one companion in the same PNR. If you reserved separately, I don't think you can do it for anyone. I see you're also a UA elite - it's similar to the rules for premiers bringing a companion into E+, except that you don't get any more legroom in the preferred seats on NW.

Also - you can't get the Coach Choice seats until OLCI at 24:00 out from departure time, no matter what your status. So you'll need to be quick on the trigger to bag them.

I know the seat maps at seatguru say that NW seat pitch is the same as UA in Economy Minus, but in my experience the NW coach seats on the 319/320 they usually fly on the DEN-MSP route have a bit more pitch than UA in E- on the same kind of bird. Not as good as E+, but reasonably comfortable in coach. At least it was yesterday on that route.