Okay, I figured it out. The trick was that you can't access it via "ldap://" it has to be accessed via "WinNT://Domain/group_name,group". where "Domain" is the domain your AD is serving and "group_name" is the name of the group you want. In this case, it is "Domain Guests" to add and "Domain Users" to delete.

Once that is opened, you can use the normal ADSI commands to add the user to it.