Adding a role for user after registration in MVC 5

If you're using the built in authentication manager and Account Controller then this will do it:

UserManager.AddToRole(user.Id, "<Role>");

You need to use UserManager to add role to the user. See below:

    [HttpPost]
    [AllowAnonymous]
    [ValidateAntiForgeryToken]
    public async Task<ActionResult> Register(RegisterViewModel model)
    {
        using (var context = new ApplicationDbContext())
        {
            if (ModelState.IsValid)
            {
                var user = new ApplicationUser() { UserName = model.UserName };
                var result = await UserManager.CreateAsync(user, model.Password);

                var roleStore = new RoleStore<IdentityRole>(context);
                var roleManager = new RoleManager<IdentityRole>(roleStore);

                var userStore = new UserStore<ApplicationUser>(context);
                var userManager = new UserManager<ApplicationUser>(userStore);
                userManager.AddToRole(user.Id, "User");

                if (result.Succeeded)
                {
                    await SignInAsync(user, isPersistent: false);
                    return RedirectToAction("Index", "Home");
                }
                else
                {
                    AddErrors(result);
                }
            }
            // If we got this far, something failed, redisplay form
            return View(model);                
        }
    }