Thanks to the awesomeness of dA groups, there's now an automatic submission limit. I figured that each member or deviant may submit 2 deviations per day. Does that sound fair? :3
Also, I noticed on many groups that members are submitting non-member fanart, and I'm fine with that as long as the original artist agrees - if they don't, just don't pursue it, because obviously they don't want their work in the group. If you happen to be passing by and you're not a member, but you'd like to share your work with the gallery, you may submit it to the group, too.