Yes I think about these things eight months in advance.
I need group costume ideas for like around six people. We considered Star Wars characters but I think I'm going to have trouble finding an affordable Jango Fett costume gonk
You could try Avatar the last airbender. There's plenty to choose from, though I think the number is even of boy and girl characters if you go for the main group. As in, Aang, boy, Zuko, boy, Sokka, boy, Katara, Girl, Suki, girl, and Toph, girl. It's just a suggestion, but at least it has six and most of the costumes are simple enough to make depending on which one you choose.