Other great resources: Official JS API docs, Scripting Forum
DORDER_DISEMBARK
order a droid to disembark from a DROID_TRANSPORTER (i think?)
currently there's no way to tell if a droid is on a transport (you'll have to remember from the point where you tell droid to DORDER_EMBARK), and there's no way to know if a transport is landed or not (I think they need to be landed before droids can leave them)