If you want people to reply to an invitation, put (RSVP) at the bottom.

如果想让人回复邀请函，就要在邀请函底下注明尽快回复字样(RSVP)。

youdao

For example, if you send me an invitation to speak at a conference and I ask what the topic is, you might reply with just the topic, snipping out all the details of the conference.

举个例子，如果你给我发送一封邮件，邀请我在一个会议上发言，我问发言的主题是什么，你可能只回复我这个主题，剪去了所有关于这个会议的细节。

youdao

An invitation is a courtesy and must be promptly acknowledged. Whether a reply is called for or not, one must advise either an acceptance or refusal. A host should know how many guests to expect.

不论答复是接受还是拒绝，邀请都必须立即确认，以便主人了解所需接待客人的人数。

youdao

The Iranian government also did not reply to an Italian invitation to participate in talks on the sidelines of the G-8 foreign ministers meeting later this week in Trieste.

VOA: standard.2009.06.24