When a Chinese is asked the question, "What nationality do you belong to?"
The person will reply, "I am a Chinese" Here an indefinite article is used.
In any communication, whey you say, "To us", more than one person is referred to, and you cannot use an indefinite article which is signifying one in number.
As such, you drop the article.
The use of the in the other sentence is the reference is to a specific and unique people.