To add Login with Slack functionality to an app using Node and WorkOS, install the WorkOS Node SDK, set up secrets such as API keys and client IDs, configure a Slack connection by getting a Redirect URI from the WorkOS dashboard and setting it in the Slack OAuth app, provide client credentials to WorkOS, configure a redirect URI, set up a frontend with React and add code to handle user authentication, set up a backend using Express, direct users to sign in or sign up using AuthKit, extract the authorization code, exchange it for an access token, and use the user's profile information. This is just the first step in identity management and involves handling the user session, implementing logout, adding SAML SSO, implementing access control, provisioning users automatically, handling failed authentication events, and more.