You're both partially right. One MOA is an absolute. As sensop said 1 MOA at 100 yds is 1.047" (sine of 1 min. of 1 deg. X 3600 inches). To figure the MOA of your measured group, simply divide the group size by the MOA size at the distance you are shooting. For example, if you shoot a .66 group at 100 yds, that would be .66 / 1.047 = .63 MOA.
It's easy to figure MOA at common shooting distances based on the 100 yd figure by simple multiplication or division. You only have to remember one number: 1.047. At 50 yds, it's 1.047 / 2 = .52", at 300 yds it's 1.047 X 3 = 3.14". So if you shoot that .66" group at only 50 yds, it's .66 /.52 = 1.27 MOA.