GNPA and CNPA members have an annual winter trip to Yellowstone. It's a small number of members as you describe - and open to non-members. I've been on this trip in earlier years and it is excellent. When I was part of the trip, I flew in a couple of days earlier, stayed in Gardiner and spent some time in Lamar Valley using my own rental car.
Here is their information:
Bob Waters of the CNPA and the GNPA’s Chris Dekle will again host a Winter, 2015 trip to Yellowstone National Park. You won’t want to miss it. Our past 3 trips have been loved by all who went.
DATES in Park: 5 Full Days in Yellowstone! January 31-February 4 (return to Bozeman on the 5th) and a second trip, Feb 6-10 (return to Bozeman on the 11th), 2015
Includes everything except Your travel to Bozeman, MT, food or park entrance fee and meals. All fees, transportation, guides, private snow coaches, hotel, are all included.
All for just $1,870.
Only a few spaces remain so move fast and join us.
OPEN TO MEMBERS AND NON-MEMBERS
Full information and sign up for the trip here (http://tinyurl.com/yellowstone2015