Watch: How to Add and Share your form with Collaborators in Google Forms
If you're working on a form and would like to share it with a collaborator, click the File and select Share. Then, from the "Sharing settings" dialog, you can specify individual collaborators with whom you'd like to share your form for editing. To add a collaborator, begin typing his or her name in the "Add people" text box.
When you're working with a collaborator on a form, each of you can edit one input field at a time. For example, your collaborator can edit the text of a question while you edit the help text of that same question.