CA Productions - Secret co-developer who worked on TONS of Hudson games with graphic design and programming. They have been involved in almost all the Mario Party and Wii Party games as a sub-developer (almost full co-developer really). They continued working on them with NdCube.
Smileboom - As of 2008, they joined and became one of the lead co-developers assisting on Mario Party and Wii Party.
T's Music - Nintendo has hired T's Music before on some outsourced 1.5 party projects, but they have worked on all the Nd Cube party games for music and sound.
Monegi - Initially also assisted on several Mario Party games. They also developed some N64 tools / demos. While the company was a Nintendo / Hudson joint-venture, I know for a fact some of the programmers (Hiroshi Yoshida ex) were from the Nintendo side. Nintendo transferred some Monegi staff over to ND Cube in 2001.
I will be updating this with more developers.