The quartet were rent asunder by dissension
What is the meaning of "rent asunder by dissension"?

[
I looked around. Most of the remaining women were now having
fights with men said to be their husbands. Even Jordan’s party, the
quartet from East Egg, were rent asunder by dissension.
]

Ch.3
http://ebooks.adelaide.edu.au/f/fitzgerald/f_scott/gatsby/complete.html#chapter3

Question

The quartet were rent asunder by dissension
What is the meaning of "rent asunder by dissension"?

[
I looked around. Most of the remaining women were now having
fights with men said to be their husbands. Even Jordan’s party, the
quartet from East Egg, were rent asunder by dissension.
]

Ch.3
http://ebooks.adelaide.edu.au/f/fitzgerald/f_scott/gatsby/complete.html#chapter3

Answer

Asunder = apart
Quartet = a group of four
dissension = strong disagreement; a contention or quarrel; discord. 
Rend asunder = tear apart

I looked around and saw the most of the remaining women were now having fights with their husbands. Even Jordan's party (a group of four from East Egg) were torn apart because of disagreements / quarrels.

Answer

Even Jordan's group of four from East Egg, were split apart by disagreements and arguments.

If you rend something asunder, you tear it apart.