You can see the list of maintainers and testers here:
http://community.linuxmint.com/iso/team. The maintainers are the ones putting together the editions of Linux Mint, with Clem and Fred being the primary maintainers. This just covers the release of new editions, and not the development of any software that is part of those editions. MATE, for example, has its own team (
http://mate-desktop.org/team/). For the software developed by the Linux Mint developers, with Clem as lead developer, there are a lot of contributing developers. Especially Cinnamon has a many awesome developers involved (
https://github.com/linuxmint/Cinnamon/network).