Well, on the submission page, there's the "Submit to a Group" box right under the title & category. All of the groups you're an admin on should be listed there with checkboxes. You check off the group(s) you want to submit to, and a drop-down menu with the various folders in the group's gallery should appear. Choose the folder you want. There's also a checkbox under that for "Submit exclusively to this group". If you check that off, it will be submitted only to the group gallery and won't be included in your gallery; if you want a copy in your gallery, you have to submit it again. If you leave that unchecked, though, it will be submitted to your gallery and automatically also show up in the proper folder in the group gallery.