ObtenerUsuariosConRol.cs

using (ApplicationDbContext db = new ApplicationDbContext())
            {
                var rm = new RoleManager<IdentityRole>(new RoleStore<IdentityRole>(db));
                var roleId = rm.FindByName("NombreDeLRol").Name;
                var usuarios = db.Users.Where(x => x.Roles.Any(y => y.RoleId == roleId)).ToList();
            }