I personally feel that a language core should be the work of a single person. Period.
The core set of axioms that form a language are almost fundamental as the laws of nature to that language. Just imagine theory of relativity being developed by several scientists together... almost impossible.
Even 2 persons should not work together on a language core. The quantity of communication required if 2 persons work on a single language is so large, that hinders and messes up the development process.
If there's one place where the saying "Too many cooks, spoil the soup" applies; its core language design.