ClientID
PartnerClientKey
UserID
PartnerUserKey
CustomerKey

public void testAddUserToAccount()
{
    Account account = new Account();
    Account.ID=12334;
    if (account != null)
    {
        AccountUser accountUser = new AccountUser();
        accountUser.Name = "ACRUZ";
        accountUser.UserID = "ACRUZ";
        accountUser.IsAPIUser = true;
        accountUser.IsAPIUserSpecified = true;
        accountUser.IsLocked = false;
        accountUser.IsLockedSpecified = true;
        accountUser.Password = "XXX";
        accountUser.MustChangePassword = false;
        accountUser.MustChangePasswordSpecified = true;
        accountUser.Email = "acruz@example.com";
        UserAccess access = new UserAccess();
        //3 CLIENT_ADMIN Add Users to Account
        //4 PRO_ADMIN Create/View Accounts
        access.ID = 3;
        access.IDSpecified = true; //.Net specific
        accountUser.UserPermissions = new UserAccess[] {access};
        //This tells that create user in subaccount
        ClientID clientID = new ClientID();
        clientID.PartnerClientKey = "12345";
        clientID.IDSpecified = true;
        accountUser.Client = clientID;
        APIObject[] apiObjects = {accountUser};
        String requestId = null;
        String overAllStatus = null;
        CreateResult[] results = partnerAPIWse.Create(new CreateOptions(), apiObjects, out requestId, out overAllStatus);
        if (results != null)
        {
            foreach (CreateResult result in results)
            {
                Console.WriteLine();
                Console.WriteLine("Status Message ### " +
                result.StatusMessage);
            }
        }
        else
        {
            Console.Write("Error ...... ");
        }
    }
}