When to use to join in Korean
Korean: 가입하다[ ga-ip-ha-da ]

When to use it
가입하다 means to join or sign up for a group, service, club, or membership. You use it when someone becomes a member of something, like a website, gym, or school club. In Korean, it is often used for memberships and subscriptions, not for simply going with friends somewhere.
Example sentences
저는 학교 동아리에 가입했어요.
I joined a school club.
엄마는 헬스장에 가입했어요.
Mom joined a gym.
이 앱에 가입하려면 이메일이 필요해요.
You need an email to sign up for this app.
우리는 여행자 보험에 가입했어요.
We signed up for travel insurance.
형이 새 인터넷 서비스에 가입했어요.
My older brother signed up for a new internet service.
저는 반 단체 채팅방에 가입했어요.
I joined the class group chat.