Very basic implementation of a Slack SSO plugin.

slack-sso.php

<?php
/*
Plugin Name: Slack SSO
Plugin URI: https://gist.github.com/rheinardkorf/91431d6e8e7b5070730352321876d5d3
Description: Sign into your WordPress site with Slack.
Version: 0.1-alpha
Author: Rheinard Korf
Author URI: https://gist.github.com/rheinardkorf/91431d6e8e7b5070730352321876d5d3
License: GPL2
License URI: http://www.gnu.org/licenses/gpl-2.0.html
Text Domain: slack-sso
Domain Path: /languages
*/

const SLACK_SSO_CLIENT_ID     = '<YOUR_SLACK_CLIENT_ID>';
const SLACK_SSO_CLIENT_SECRET = '<YOUR_SLACK_CLIENT_SECRET>';

/**
 * Handle Slack Sign-In.
 */
add_action( 'init', function () {

	/**
	 * Note: Set your callback URL to your login page and include ?slack_sso=1 as the URI parameter for the following to work.
	 **/
	
	$is_slack_sso           = isset( $_GET['slack_sso'] );
	$is_slack_error         = isset( $_GET['error'] );
	$is_slack_authenticated = isset( $_GET['code'] );
	$error_code             = $is_slack_error ? sanitize_text_field( wp_unslash( $_GET['error'] ) ) : '';
	$auth_code              = $is_slack_authenticated ? sanitize_text_field( wp_unslash( $_GET['code'] ) ) : '';

	if ( $is_slack_sso && $is_slack_authenticated ) {

		/**
		 * User approved Slack Sign-In.
		 */
		$slack_auth_uri  = esc_url_raw( sprintf( 'https://slack.com/api/oauth.access?client_id=%s&client_secret=%s&code=%s', SLACK_SSO_CLIENT_ID, SLACK_SSO_CLIENT_SECRET, $auth_code ) );
		$response        = wp_safe_remote_get( $slack_auth_uri );
		$response_object = json_decode( $response['body'] );

		if ( isset( $response_object->user ) ) {
			$slack_user = $response_object->user;
			$user       = get_user_by( 'email', $slack_user->email );

			if ( false === $user ) {
				$id   = wp_create_user( $slack_user->name, wp_generate_password(), $slack_user->email );
				$user = ! is_wp_error( $id ) ? get_user_by( 'email', $slack_user->email ) : false;
			}

			if ( false === $user ) {
				$is_slack_error = true;
				$error_code     = 'wp_user_error';
			} else {

				// Save slack user to user meta.
				update_user_option( $user->ID, 'slack_user', wp_json_encode( $slack_user ) );

				wp_set_auth_cookie( $user->ID );

				if ( isset( $_REQUEST['redirect_to'] ) ) {
					$redirect_to = $_REQUEST['redirect_to'];
					// Redirect to https if user wants ssl
					if ( is_ssl() && false !== strpos( $redirect_to, 'wp-admin' ) ) {
						$redirect_to = preg_replace( '|^http://|', 'https://', $redirect_to );
					}
				} else {
					$redirect_to = admin_url();
				}

				wp_safe_redirect( $redirect_to );
				exit();
			}
		}
	}

	/**
	 * Well this sign-in was unsuccessful.
	 */
	if ( $is_slack_sso && $is_slack_error ) {
		switch ( $error_code ) {
			case 'wp_user_error':
				$error_message = esc_html__( 'Could not find or create a user.', 'slack-sso' );
				break;
			case 'access_denied':
				$error_message = esc_html__( 'Could not log you in via Slack: Access Denied.', 'slack-sso' );
				break;
			default:
				$error_message = 'wat';
		}

		add_filter( 'login_message', function ( $message ) use ( $error_message ) {
			return '<div id="login_error">' . apply_filters( 'login_errors', $error_message ) . '</div>';
		} );
	}
} );

/**
 * Add "Sign in with Slack" button on Login Form.
 */
add_action( 'login_form', function () {
	echo sprintf( '<div><a href="https://slack.com/oauth/authorize?scope=identity.basic,identity.email,identity.team,identity.avatar&client_id=%s"><img alt="Sign in with Slack" height="40" width="172" src="https://platform.slack-edge.com/img/sign_in_with_slack.png" srcset="https://platform.slack-edge.com/img/sign_in_with_slack.png 1x, https://platform.slack-edge.com/img/sign_in_with_slack@2x.png 2x" /></a></div>', SLACK_SSO_CLIENT_ID ); // PHPCS: XSS ok.
} );