They are friends because they sat at a table together?

Yep. The only ones that seemed like friends were Alex and Sarah. Everyone else either hated each other or were sexually involved in some way.

That's how Alex joined the group. Everyone else were already friends with one another and then Ben invited Alex to join them. That is the reason why Alex is so close to Ben.

It seemed to me that they all met in college and that scene in their dorm cafeteria was just a moment very early in the group's friendship that Alex was introduced to the group, and the others had just barely known each other yet. If you went to a college where you barely knew anyone, you would probably understand the scene better. Then, after college, the group grew apart.