ASP.NET MVC 5 Web apps that use ASP.NET Identity 2.0 with OWIN as described at http://www.asp.net/identity can use Slack for user identity and sign in.
This library was originally developed for and is in use at https://freebusy.io
This library is available as a NuGet package at https://www.nuget.org/packages/Owin.Security.Providers.Slack/
See an example of how to configure your ASP.NET web app at https://github.com/AreYouFreeBusy/Slack-OAuth-Provider/blob/master/Slack-OAuth-Demo/App_Start/Startup.Auth.cs