Adding a Reviewer to a GitHub Pull Request

Since Dec. 2016, there is a more integrated way to notify the people you want for reviewing your PR.
See "Introducing review requests"

You can now request a review explicitly from collaborators, making it easier to specify who you'd like to review your pull request.

You can also see a list of people who you are awaiting review from in the pull request page sidebar, as well as the status of reviews from those who have already left them.

As noted in the comments by Alexander Amelkin:

This feature is only for the owner of the repository, not for the author of the pull request.
The pull request author still can't request anyone to review his request.
The only way is to mention the person in a comment.

Yes, in Github you only can assign a PR to one person. And if you want to notify more people, mention them in the comments.