Mailing lists and IRC
GHC developers hang out on the ghc-devs mailing list.
There is also ghc-tickets, which receives a copy of traffic from the bug tracker, ghc-commits, which receives a copy of all commit messages, and ghc-builds, which receives reports from the automatic nightly builds.
The traffic levels vary quite a bit depending on how much activity there is.
GHC developers also hang out on #ghc channel on FreeNode. This is a good place to ask questions related to working on GHC itself; if you have more general questions then #haskell is probably more appropriate.