Use values like <em>user_email</em>, <em>user_login</em>, <em>display_name</em>, <em>user_nicename</em>, <em>user_url</em>, <em>user_registered</em> or <em>include</em>. You can sort by a usermeta field simply using the value <em>user-field-xxx</em> where xxx is the user field slug.