Translation:They know each other.
What is the difference between 그들 and 그휘?
Generally if you want to ask or say that someone knows someone else, you would use the term "아는 사이인가요/예요?"
Is 'they' gendered?
그들은 알고 지냅니다.
그들은 서로 알고 지냅니다.
These are more natural, I think.