You can go to your deviation you want to submit and on the right side there should be a little button that says "Add to a Group". Click that and pick the bubble that says "Type a Group name." After you type in the name of our group, pick the correct folder and click the submit button.
OR
You can go to the group profile and click the button that says "Submit Art" at the top. You can upload your deviation like that and it should automatically be put into the group. (Always make sure it's put in the right folder though)