These are the v2 docs. For the current version, look here.

users

The GitHub users who are in this group. Case-insensitive.

Can be either a list of usernames, or the string "all" to automatically include all collaborators of the repo.

A reviewer group must use either users or teams, but not both.

Example usage:

groups:
  designers:
    users:
      - githubusernameone
      - githubusernametwo
      - githubusernamethree
groups:
  designers:
    users: all  # all repo collaborators