Ah, now I'm not quite sure why that is. It's a bit of a pain, really. But no matter, you can still contribute art to the group. It just takes a little longer is all.
All you have to go is go into the group's gallery and on top of the featured folder preview it should have a button that says: "Submit art to this gallery". Click on that. It should then bring up another couple of options and one of them will say something along the lines of: "Contribute existing deviation." Just select that and it should be smooth sailing from there.
Sorry about this.
But I should thank you for pointing this out to me. We haven't been getting many submissions lately and I bet that's part oft the reason why. I'll do a group journal at some stage, explaining the contribution process.
Anyway, sorry for the inconvenience. I hope this helps.
(BTW, sorry it took me so long to reply!)