by invitation

基本解釋

Alistair: Because the event itself is small – we could only accommodate 30-35 people, the event was by invitation.
因為這次活動本身很小——我們只能為30-35人提供食宿，所以活動是邀請制的。
The baseline entrance level is only available by invitation; access to sensitive information is restricted solely to those users who have attained the highest levels.
入門級別的訪問權限需要邀請才能獲得。只有擁有最高級別訪問權限的用戶才可以訪問敏感信息。
Conversations can be public, private or by invitation only (though still visible to the public).
對話可以是公開的、秘密的，或者只能通過邀請才能加入。

It has similarly failed to respond to an invitation by European Union Foreign Policy chief Javier Solana for a new round of talks on its nuclear program with the five permanent U.N.Security Council member countries and Germany,the P5+1.
They've got the gardens open by invitation.
他們有花園，對受邀者開放。
快樂的休息日下午 - SpeakingMax英語口語達人
you could leave your response to the invitation slightly open-ended by saying you're not sure.
你可以表示你不確定，開放性地回復這個邀請。
I don't know 課堂 - SpeakingMax英語口語達人

IR—Peter Drucker's comments on financial services (By invitation, September 25th) dismiss derivative products much too quickly.
ECONOMIST: Letters