Apparently it's impossible to submit to groups too. When you submit, it'll say the submission was accepted, but it won't appear in your group messages or the group's queue.
I'm really hoping it's just a caching issue and the submissions actually went through. Otherwise I just wasted about an hour of work on a group I moderate, and lost a bunch of pics since I removed them from my to-submit-to-group collection as I went. I'm a little pissed off right now.
It's definitely not just me, either, because I haven't gotten any submissions from groups in my devwatch inbox in the last hour even though I'm a member of some high-volume groups, but I have gotten some from individual artists (so it's not my inbox not updating).
Of course this happens on a weekend when there's nobody around to fix it.