I wrote something (long paragraph about this) but can't seem to find it. Basically positions you'll need are:
-Coder
-Animator
-Mapper
-Modeller/Skinner
-2D Artist
-Sound Artist
-Website Manager
-Leader
Some people can mix and match (e.g. be both an animator and a modeller/skinner). But before you start a mod, you need a concept of how you are going to do it or else people won't join (the more organized you are the more people are going to want to not only join your mod but do work as well).