Notice how Selcen wrote "Misafirler(i)" and not "Misafirler", this is another clue, because in the example she gave, the presence of the accusative marker gives away the state of the word being a definite object. In the example Duo gave there was no marker, because it's the subject, and subjects don't take markers.
That would be 'Misafirler nerede oturuyor olacak?'.
'Misafirler nerede oturacak?' would be 'Where will the guests sit?' again. But here 'sit' would be in the stative form whereas in the given sentence it is in the dynamic form. Same rule applies to the Turkish versions. To be clear, 'to be seated/be sitting' could be used.