"他们很开朗"的英文表达可以是 "they are outgoing" 或者 "they are extroverted."

"Outgoing" 这个词的来源是源自中世纪英语 "outga(i)ng," 它有 "出去" 的意思。现在，"outgoing" 常用来形容一个人性格开朗、乐于助人、爱交际。

"Extroverted" 这个词来自拉丁文 "extrovertere," 它有 "向外" 的意思。现在，"extroverted" 常用来形容一个人性格外向、乐于助人、喜欢社交活动。

下面是一些例句：

She's very outgoing and always makes new friends wherever she goes. (她非常开朗，无论走到哪里都会交到新朋友。)
He's quite extroverted and enjoys sociapzing with others. (他很外向，喜欢与人交往。)
They are both outgoing and love to meet new people. (他们都很开朗，喜欢认识新朋友。)