Given that atheist groups in Idaho aren't all that well published here (or elsewhere) I figured it wouldn't hurt to list a few. I hang around West Valley Free Thinkers for the most part (brace for blatant plug); we meet at the Flying M in Nampa: http://www.meetup.com/West-Valley-Free-Thinkers/
There's a fair bit of cross-membership between the groups. The numbers that show up for these groups ranges from something like 4-20 people for typical meetings, with some events drawing around 60-80. All of these groups are less than 2 years old, but appear to be growing very quickly.
If I've missed any existing groups, or you decide to start your own, you can post it here.
I noticed that http://unitedcor.org/ is doing a nationwide billboard campaign and that there is not an Idaho affiliation. I would suspect that maybe the more connected folk in Boise should contact them ans see if they might be willing to assist in passing along the word.