Have all of the authors together draft an outline of the novel. Make sure that all the authors agree on the major plots points, characters, obstacles and resolution. Give all authors involved a copy of the outline.
Decide how the writing will be divided. Some writing teams divide novel writing by assigning chapters or parts of chapters to specific authors, some authors write strictly for a certain character, others divide the novel into major sections and assign a section to each writer.
Stick to the portion of the novel that you have been assigned and make sure that the other author(s) are also writing diligently. You can establish weekly meetings where the writing team members summarize or read what they have written thus far, so that everyone is kept abreast of the progress of the book.
Read the first draft aloud when finished. Make sure that all the authors are there, taking notes on any shortcomings of the book or things that they need to change. Initiate a roundtable discussion after the reading.
Work vigilantly on the portions of the book that you are responsible for, revising them as needed. Make sure that the other authors are also revising diligently.
Gather all the authors together and read the second draft aloud. Discuss what elements, if any, need to be changed or tweaked. Assign parts for writers to revise.
Read all the revisions and discuss the book's current state with all the authors. Decide together if further drafts are necessary or if the novel is acceptable in its current state.