If your using just open source so long as he is in the same Base DN as the rest of your Graylog users all he needs is to log in the interface. If this user was just added to the domain and he/she is in the same DN then maybe refreshing your AD/LDAP might be an option.
NOTE: I probably should have ask this but was this person added to the domain account or was just added on the Graylogs Web UI? If added to the web interface maybe check there account is correct.