From My Surveys, select your survey by clicking on it from the list displayed in My Surveys or My Team Surveys. Then click on the Edit button, followed by 'Advanced', then 'Group' buttons.
To assign a Group to a question, click on the 'Grouping' button which is displayed next to each question in your survey. You can create a new group by typing in its name or choose an existing group from the drop down box. Select the Group to which you'd like to assign the question, or create a new Group and click on OK. The name of the group appears at the side of each question.