Let's do some logic, children!
We have three sisters; Feyre, Nesta, Elain
And three brothers; Rhysand, Cassian, Azriel
Each sister has two friends, which makes them three too!
Feyre has Morrigan and Amren,
Nesta has Emerie and Gwyneth,
Elain has Nuala and Cerridwen
These friends are connected to each brother who pairs with each sister!
Feyre makes pair with Rhysand! And her friends are the members of the Inner Circle and Second-In-Command and Third-In-Command to the High Lord of the Night Court!
Nesta makes pair with Cassian! And her friends are the Valkyrie warriors trained by the Illyrian General Commander of the Night Court armies!
Elain makes pair with Azriel! And her friends are the spies of the Night Court trained by the Spymaster of the Night Court!