by invitation

基本解释

Alistair: Because the event itself is small – we could only accommodate 30-35 people, the event was by invitation.
因为这次活动本身很小——我们只能为30-35人提供食宿，所以活动是邀请制的。
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.
入门级别的访问权限需要邀请才能获得。只有拥有最高级别访问权限的用户才可以访问敏感信息。
Conversations can be public, private or by invitation only (though still visible to the public).
对话可以是公开的、秘密的，或者只能通过邀请才能加入。

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.
他们有花园，对受邀者开放。
快乐的休息日下午 - SpeakingMax英语口语达人
you could leave your response to the invitation slightly open-ended by saying you're not sure.
你可以表示你不确定，开放性地回复这个邀请。
I don't know 课堂 - SpeakingMax英语口语达人

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