I used the example in this page to add a user to an Active Directory group, but I get an exception with the message "Server is unwilling to process the request" when executing
dirEntry.Properties["member"].Add(userDn);
I had a similar issue where I was trying to add a member to a group. Specifically trying to add a group to a group and getting the same helpful error 'The server is unwilling to process the request' The answer provided by the OP did not work for me.
For me, the reason I was unable to add a group to my group was because the group I was trying to add members to was a 'global' scoped group whereas it needed to be a 'universal' scoped group. Hope this helps someone.