_LoginPartial.cshtml

@using Microsoft.AspNetCore.Identity

@inject SignInManager<StudentUser> SignInManager
@inject UserManager<StudentUser> UserManager

@if (SignInManager.IsSignedIn(User))
{
    <form asp-route="loguit" method="post" id="logoutForm" class="navbar-right">
        <ul class="nav navbar-nav navbar-right">
            <li>
                <a asp-area="Identity" asp-page="/Account/Manage/Index" title="Manage">Hello @UserManager.GetUserName(User)!</a>
            </li>
            <li>
                <button type="submit" class="btn btn-link navbar-btn navbar-link">Loguit</button>
            </li>
        </ul>
    </form>
}

ApplicationUser.cs

public abstract class ApplicationUser : IdentityUser
{
    [PersonalData]
    public string FirstName { get; set; }

    [PersonalData]
    public string LastName { get; set; }

    [NotMapped]
    public string FullName => $"{FirstName} {LastName}";
}

EmployeeUser.cs

public class EmployeeUser : ApplicationUser { }

Startup.cs

public class Startup
{
    #region Public Properties

    /// <summary>
    /// The application's configuration.
    /// </summary>
    public IConfiguration Configuration { get; }

    /// <summary>
    /// Information about the current environment.
    /// </summary>
    public IHostingEnvironment Environment { get; }

    #endregion

    #region Private Properties

    /// <summary>
    /// The scoped logging service
    /// </summary>
    private ILogger logger;

    #endregion

    #region Constructor

    /// <summary>
    /// Main entry point for start of web server
    /// </summary>
    /// <param name="configuration"></param>
    /// <param name="environment"></param>
    /// <param name="loggerFactory"></param>
    public Startup(
        IConfiguration configuration,
        IHostingEnvironment environment,
        ILoggerFactory loggerFactory
    )
    {
        Configuration = configuration;
        Environment = environment;

        // Create a logger for the startup class
        logger = loggerFactory.CreateLogger<Startup>();
    }

    #endregion

    // This method gets called by the runtime. Use this method to add services to the container.
    public void ConfigureServices(IServiceCollection services)
    {
        // Change the cookie policy
        services.Configure<CookiePolicyOptions>(options =>
        {
            // This lambda determines whether user consent for non-essential cookies is needed for a given request.
            options.CheckConsentNeeded = context => false; // No cookie warning
            options.MinimumSameSitePolicy = SameSiteMode.Lax;
            options.Secure = CookieSecurePolicy.Always;
        });

        // Register the Application database context
        services.AddDbContext<ApplicationDbContext>(options =>
            // Use Microsoft SQL Server
            options.UseSqlServer(
                // Grab the connection string from "appsettings.json"
                Configuration.GetConnectionString("DefaultConnection")));

        // Add the identity service
        services.AddIdentity<ApplicationUser, IdentityRole>()

            // Adds UserStore and RoleStore from this context
            // that is consumed by the UserManager and RoleManager
            .AddEntityFrameworkStores<ApplicationDbContext>()

            // Adds a provider that generates unique keys and hashes for things like
            // forgot password link, change email etc...
            .AddDefaultTokenProviders();

        // >> I tried this answer, but that didn't work unfortuantly (see answer below)
        services.AddScoped<UserManager<StudentUser>>();
        services.AddScoped<UserManager<EmployeeUser>>();

        // Add MVC
        services.AddMvc()
            // Keep using .NET Core 2.1
            .SetCompatibilityVersion(CompatibilityVersion.Version_2_1);

        // Add the SignalR services
        services.AddSignalR(options =>
        {
            // Only enable detailed errors when
            // the application runs in a development environment
            options.EnableDetailedErrors = Environment.IsDevelopment();
        });
    }

    // This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
    public void Configure(IApplicationBuilder app, IHostingEnvironment env)
    {
        // When the application runs in a development environment...
        if (env.IsDevelopment())
        {
            // Show detailed exception pages
            app.UseDeveloperExceptionPage();

            // Show database errors
            app.UseDatabaseErrorPage();
        }
        else
        {
            // Use a general error page
            app.UseExceptionHandler("/Home/Error");

            // Force the webserver to only accept HTTPS requests
            app.UseHsts();
        }

        // Redirect HTTP to HTTPS
        app.UseHttpsRedirection();

        // Serve static files from /wwwroot
        app.UseStaticFiles();

        // Add a cookie policy middleware
        app.UseCookiePolicy();

        // Setup SignalR
        app.UseSignalR(routes =>
        {
            routes.MapHub<ClassroomHub>("/hubs/classroom");
        });

        // Enable user authentication
        app.UseAuthentication();

        // Setup MVC routes
        app.UseMvc(routes =>
        {
            // Default route pattern
            routes.MapRoute(
                name: "default",
                template: "{controller=Home}/{action=Index}/{id?}");
        });
    }
}

StudentUser.cs

public class StudentUser : ApplicationUser
{
    [PersonalData]
    [Required]
    public string StudentNumber { get; set; }

    // A user belongs to one group
    public Group Group { get; set; }
}