I have a friend who is a member here. Let me speak with him first then I'll decide if I'm going to join or not.
この場合、次のような言い方ができますよ。
ーI have a friend who is a member here. Let me speak with him first then I'll decide if I'm going to join or not.
「ここの会員になっている友達がいるので、まず彼と話させて。それから入会するかどうか決めます」
to decide if I'm going to jin or not で「入会するかどうか決める」
ご参考まで!