Use tables. Give each sub group or student a section to work on. Try sharing a presentation rather than a document. Ask student to write on a specific slide number. Either that, or use Google Forms and display the spreadsheet with all the contributions. Another option is Doctopus, a script that will allow you to create project groups rather than whole class assignments.
Although I love Google Drive, I am having an issue with students working together in docs. When they work on a group project and multiple people are editing at the same time, the text and such constantly changes positioning. This gets very difficult to type and can be confusing, any help or thoughts?