When you select your file for upload, you can scroll past the Title and Description boxes. Scroll past the Category section. Now you're at Options. You have options to submit to folders on your own page, and below that box is a box containing all of the groups to which you belong.



Come to think of it, that might just be an admin privilege.



If it is, and you can't do it like that, you'll have to just come to the deviation address, like the one on which this comment is posted. Scroll down past the "More from Deviantart" box you see on the right side. There is a "+ Add to a Group" button.



And finally, you can go to the desired group, then to the desired folder, and go into it. From there, you'll find a button on the right side saying, "Contribute to this folder".