I Assume you want to give option of registration to some particular role, so here are the steps you need to follow
First create a role, then Assign the role to those users you want to allow and Last you need to put Authorization on all those actions.
For creating a role you can use Role Manager,
var roleManager = new RoleManager<IdentityRole>(new RoleStore<IdentityRole>(new YourDataContext()));
var role = new IdentityRole(){Name="Admin"};
var result = await roleManager.CreateAsync(role);
if (result.Succeeded)
{
}
var userManager = new UserManager<AppUser>(new UserStore<AppUser>(new YourDataContext()))
var user = await userManager.FindByEmailAsync(model.Email);
await userManager.AddToRolesAsync(user.Id, model.Roles.ToArray());
[Authorize(Roles = "Admin")]
public ActionResult Register()
{
}