What does it mean? I think it's a scam, but I don't get the point

They're asking you to post this topic and they'll give you free publicity on the platform

Obviously you're right. The point is to help their tag get free publicity. The message might be batch sent to multiple users by a bot.

没有听说过有这种东西，一般邀请都是官方发的，这个账号应该就是骗子了

Rednote will do the same but you can see the official account certification at their homepage.

This is just scam.

Free to ignore.

If you just need to change a hashtag, you can give it a try. They're probably not official staff - it's likely some social media company looking for people to help make a certain hashtag trend. If they manage to make it trend, it can indeed generate more traffic.

Is it only for US users to post their slice of life?

The point is to add content to a hashtag to make it trend then their content will show up on more foryou pages because the content is "relavent".

So yes, scam. No, it doesn't really harm you. But you could be adding credibility to someone else who could be scammed.