Error adding users to roles in ADAM (A constraint violation occurred. (Exception from HRESULT: 0x8007202F)

I get the error when trying to add users to a group.

A constraint violation occurred. (Exception from HRESULT: 0x8007202F)

public string addUserToGroup(string userDN, string groupDN)
{
    try
    {
        DirectoryEntry de = new DirectoryEntry("LDAP://localhost:389/" + groupDN);
        de.Username = "myUsername";
        de.Password = "myPassword";
        de.Properties["member"].Add(userDN);
        de.CommitChanges();
        de.Close();
        //return true;
        return "yes";
    }

I'm not sure if it has something to do with the way I have set-up ADAM since I have used application directory partitions. So I have a DN of CN=sandbox,DC=ITOrg I then have 3 directory partitions like so:

CN=sandbox,CN=testApp1,DC=ITOrg
CN=sandbox,CN=testApp2,DC=ITOrg  .. etc

I have my users stored only in CN=sandbox,DC=ITOrg but I then want to give users access to 1 or more of the partitions so I was trying to add the CN of the user

CN=1@test.com,CN=People,CN=sandbox,DC=ITOrg

to this:

CN=Readers,CN=Roles,CN=sandbox,CN=testApp1,DC=ITOrg

which does not work. Although the code does work if I add the user to the Role in CN=Readers,CN=Roles,CN=sandbox,DC=ITOrg. So does that mean that I have to store users in every partition? Duplicating data?

Does anyone have any thoughts?

Typically, you'll also have to set at least the SAM Account Name on a new user entry on "straight up AD" - not sure if that applies to ADAM as well.

Marc