Who came into the room Lasaraleen and Aravis were hiding in?

the Tisroc, Prince Rabadash and the Grand Vizier

What were they discussing?  

Because the Queen Susan and her companies were all gone from Tashbaan without informing the Prince Rabadash, they were discussing how to handle this thing. Prince wanted to capture Queen Susan back as his wife. And Tisroc wouldn’t open war against Narnia to support him.

Why did the Tisroc, Prince Rabadash and the Grand Vizier meet in the old part of the palace? 

There is no council to held in the old Palace and none of the household has any occasion to come. So it’s a good place to talk about secrets.