A C2 contains only the root, 2nd, and 5th scale degrees, no 3rd.
C-D-G
I'm going to assume you're talking about the Cadd9 chord, as opposed to the C9 (C dominant 9th chord). The Cadd9 chord had the root, 3rd, 5th, and 9th scale degrees. But, you can play the 9th as a 2.
C-E-G-D or C-D-E-G