For the curious, the ones that appear here that don't, at a glance, appear to have tags/other pieces on Danbooru are:
"Nakimushi Ensemble" (the one with the white dress and headphones) "Atooijisatsu ni Teihyou no Arumi-chan" (The one with the noose) "Marygold" (The one in the hoodie, tucked next to the big Gumi's left foot. I love this song.) "Raison D'etre Choudai" (The white Gumi with the heart) "Hankouseimei" (the monochrome one near the bottom with the knife.)
If I spot any art for them within the database or any turns up, we can add their tags!