وَقِيلَ ادْعُوا شُرَكَاءكُمْ
...
And it will be said (to them): "Call upon your partners,"
meaning, `to save you from the predicament you are in, as you hoped that they would do in this world.'
...
فَدَعَوْهُمْ فَلَمْ يَسْتَجِيبُوا لَهُمْ وَرَأَوُا الْعَذَابَ
...
and they will call upon them, but they will give no answer to them, and they will see the torment.
means, they will realize for sure that they are inevitably destined for the Fire.
His saying:
...
لَوْ أَنَّهُمْ كَانُوا يَهْتَدُونَ
If only they had been guided!
means, when they see the punishment with their own eyes, they will wish that they had been among the guided in this world.
This is like the Ayah,
وَيَوْمَ يَقُولُ نَادُواْ شُرَكَأيِىَ الَّذِينَ زَعَمْتُمْ فَدَعَوْهُمْ فَلَمْ يَسْتَجِيبُواْ لَهُمْ وَجَعَلْنَا بَيْنَهُم مَّوْبِقاً
وَرَأَى الْمُجْرِمُونَ النَّارَ فَظَنُّواْ أَنَّهُمْ مُّوَاقِعُوهَا وَلَمْ يَجِدُواْ عَنْهَا مَصْرِفًا
And the Day He will say: "Call those partners of Mine whom you claimed."
Then they will cry unto them, but they will not answer them, and We shall put a Mawbiq (a barrier) between them. And the criminals, shall see the Fire and apprehend that they have to fall in it. And they will find no way of escape from there. (18:52-53)
Their attitude towards the Messengers on the Day of Resurrection
Allah says:
وَيَوْمَ يُنَادِيهِمْ فَيَقُولُ مَاذَا أَجَبْتُمُ الْمُرْسَلِينَ