What is Islam's ruling on gloating and shaming? And is it permissible for a woman to say to another, who is not yet married: "I will invite you to my wedding so that I may vex you"?

A Muslim should not gloat over his brother, but rather feel pain for what causes him pain and rejoice in what brings him joy.

It is not permissible for a Muslim to say or do anything that harms his Muslim brother, for Allah Almighty says: "And those who harm believing men and believing women undeservedly have certainly borne upon themselves a slander and a clear sin" (Al-Ahzab: 58). What is meant by harm here is verbal harm, which is a grave sin.