Yaniv was on the right track, but the magic seemed to be in "expand". Here's what ended up working:
sass: {
dist: {
files: [{
expand: true,
src: ['sitecontent/**/*.{scss,sass}'],
ext: '.css'
}]
}
}
This compiles all sass files in the sitecontent folder and all subfolders and puts the css next to the sass file - it doesn't compile them to some other folder.