A party in the Dark Maze is bound together, since its health isn't restored between levels. This means that you won't be able to break up and edit your party once you've entered the maze.
You can, however, organize a totally new party to use. You simply won't be able to use heroes from one party in another.
Up to five undefeated parties are permitted at one time. Parties that suffer defeat are removed, leaving space free for new ones.
All of the heroes in a defeated party will come down with a bad case of Maze Sickness. They'll remain out of commission until either the maze reshuffles as it does every three weeks or you start the maze over again, thereby resetting your progress.