Excuse me for not replying sooner.
Anyway, to post your drawings in groups go to the drawing in question. Then, scroll down a little bit until you see on the right (where you also see the add to favourites button, bla, bla) an option that says 'Add to a group'. Click it and then just type on the bar the group you want your drawing to be submited.
I hope it was clear!
PD.- If you're a phone user I think you just have to tap on a 'more' button and then scroll down a bit until you see that 'Add to a group' option.