Apple invented a new tag for MP3 ID3… TCMP for ID3v2.3. (It’s not in the standard.) For M4A, it’s the cpil atom. You can tell Firefly which directories you want to mark as Compilations when scanning, so you don’t need to depend on the tagging.
iTunes clients can group Compilations and hide the extra Artists (UI option), but Roku does not support this… for reasons I don’t fully understand.
Anyway, “Album Artist” metadata is how I solve grouping/browsing issues on the PC. I hope Roku and Firefly can come up with some workable solution soon… Apple’s “compilation” is far too limiting.