The game—as well as the whole Westeros.org site—is run by Elio M. García and Linda Antonsson. On the MUSH we are Balerion and Nymeria. On the A Song of Ice and Fire forum we are Ran and Linda.
We became involved with George R.R. Martin when we contacted him to ask his permission to create the MUSH. He very graciously not only gave us permission but provided us with various bits of (as of yet) unpublished information, such as additional houses and some information about the time period the game takes place in. We are now collaborating with him on the forthcoming setting guide, The World of Ice and Fire.