Попробуйте использовать DbContext.Users для вывода информации о пользователях из IdentityDbContext.cs
:
/// <summary>
/// Gets or sets the <see cref="DbSet{TEntity}"/> of Users.
/// </summary>
public DbSet<TUser> Users { get; set; }
/// <summary>
/// Gets or sets the <see cref="DbSet{TEntity}"/> of User claims.
/// </summary>
public DbSet<TUserClaim> UserClaims { get; set; }
/// <summary>
/// Gets or sets the <see cref="DbSet{TEntity}"/> of User logins.
/// </summary>
public DbSet<TUserLogin> UserLogins { get; set; }
/// <summary>
/// Gets or sets the <see cref="DbSet{TEntity}"/> of User roles.
/// </summary>
public DbSet<TUserRole> UserRoles { get; set; }
/// <summary>
/// Gets or sets the <see cref="DbSet{TEntity}"/> of User tokens.
/// </summary>
public DbSet<TUserToken> UserTokens { get; set; }
/// <summary>
/// Gets or sets the <see cref="DbSet{TEntity}"/> of roles.
/// </summary>
public DbSet<TRole> Roles { get; set; }
/// <summary>
/// Gets or sets the <see cref="DbSet{TEntity}"/> of role claims.
/// </summary>
public DbSet<TRoleClaim> RoleClaims { get; set; }
Что-то вроде:
var categories = from c in db.categories
join u in db.Users
on c.myID equals u.Id
select new { ID=c.myID, name= u.UserName};
var result = categories.ToList();