in the admin panel
on the sidebar
on the structure tab
the "Add question" button

when adding a question, there is a drop down called "Question group" for which group to put the new question in. The initially selected item in this menu is one of three cases:

1. No question/group selected: first question group of the survey
2. Question group selected: currently selected question group
3. Question selected: question group that the currently selected question belongs to

However, if you've just added a question to a group and click the "Add question" button again, the initially selected question group is the first question group of the survey, rather than the group you just added a question to even though it should be case 3 above.