Go to the group.
Click the tab: My Group Images
Click on the left: Non Group Images
Use keywords if need be to find the work you want to add.
Click add to group.
If a group isn't allowing you to add they may have stopped allowing submissions.
When you originally upload the artwork there are check boxes for any groups you have joined. Just tick the boxes for any appropriate groups. Additionally, you can 'edit' any image and do the same.
