Bellamy: you had known Bellamy on the arc and you two were close because you were the only one who knew about Octavia.
Murphy: you met Murphy during Earth skills on the arc and hit it off instantly.
Lincoln: you had met Lincoln before everyone else when you were roaming the forest and he had taken you in order to protect you.
Finn: you were best friends on the arc and grew up together.
Roan: you met Roan when he wanted to make a deal with Skaikru and took you as compensation.
Jasper: you met Jasper and Monty as the drop ship was going down to Earth but knew them a little bit back on the arc.
Monty: ^^^
Clarke: you had known Clarke on the arc as your parents worked together and you had play dates while they were working.
Octavia: you knew Octavia on the arc when Bellamy introduced you to her.
Raven: you were the first person to find Raven when she landed in the escape pod to Earth.
Lexa: you met Lexa when Clarke had tried to make a deal with trikru.