The answer here, grammatically, is easily solved by following the pronouns back to their immediate antecedent. I have highlighted these in that which follows from Luke 16:27-31:
Then I beg you, father,’ he said, ‘send Lazarus to my father’s house,
for I have five brothers. Let him warn them, so that they
will not also end up in this place of torment.’
But Abraham replied, ‘They have Moses and the prophets; let your
brothers listen to them.’
‘No, father Abraham,’ he said, ‘but if someone is sent to them
from the dead, they will repent.’
Then Abraham said to him, ‘If they do not listen to Moses and the
prophets, they will not be persuaded even if someone rises from
the dead.’”
Thus, the "they" in V31 is simply the "five brothers" of V28.
Now, how one wishes to interpret this parable is an entirely different matter. The events surrounding Jesus' telling this parable clearly show to whom He referred - the leading Jews who would not listen to the truth of the Jesus' teaching. Ample evidence of this is given in the raising of Lazarus which precipitated in the final plans of the Jews to kill Jesus, John 11:45-54.
Ellicott reaches the same conclusion in his comments on Luke 16:31 -
If they hear not Moses and the prophets.—We are accustomed, rightly
enough, to look on our Lord’s own Resurrection as leading to the great
fulfilment of these words. We should not forget, however, that there
was another fulfilment more immediately following on them. In a few
weeks, or even days, according to the best harmonists, tidings came
that Lazarus of Bethany was sick (John 11:1). In yet a few days more
that Lazarus did “rise from the dead;” cured, we may believe, of
whatever love of this world’s good things had checked his spiritual
growth, a witness of the power of Christ to raise, as from the
shadow-world of Hades, so also from the darkness of spiritual death to
newness of life. And yet that wonder also brought about no repentance,
Scribes and Pharisees, and Sadducees and priests simply took counsel
together that they might put Lazarus also to death (John 12:10). We
can hardly believe the coincidence of name and fact in this instance
to have been undesigned.