Writing Groups can be structured to provide feedback OR they can simply be a group that provides accountability by meeting to write at a regularly scheduled time. Both formats, or a combination of the two, can be effective.
Writing groups work best when working guidelines are established from the beginning: How often will you meet? How and how often will work be submitted? What are the goals for the group (i.e. focused on dissertation writing/support, job market papers & presentations, or a mixed group of students at various stages)?
1. The "shut up and write" format
2. The reading & responding format
While the feedback from members of your dissertation committee is crucial and a primary concern, a writing group can provide regular feedback on your work (unfinished and in progress) in an informal environment.