@SharonOu
To make sure we are talking about the same thing, you have gone to the People page and clicked on +People. Then you are trying to add user(s) by Email address.
The first thing that comes to mind is what delimiter are you using between the email addresses? If you are copy/pasting the list from somewhere, it may not use commas or put them on a new line, which is required for this to work. If the list is coming from Outlook, for example, it puts semicolons between email addresses.
Does it work if you enter just one email address? If so, then it's probably a delimiter issue. If not, it may be an issue with the email. Canvas supports international students and UTF8 characters, so I would check the delimiter first.