Because Kyungsoo's born early in the year i.e January, he can be friends with the people born in the year before him. So Baek, Chen and Yeol become his friends. Kai, who was also born in January is friends with Kyungsoo but all the others are hyung to him. Kai's best friend Taemin was born a year before Kai but cuz Kai was born in Jan they become friends. Im the same, born in Jan so if I go to Korea, I can befriend the people born a year before me as well as the people born in the same year as well. Another example is SuJu's maknae Kyuhyun who"s friends with Ryeowook cuz he was born in Feb^^