In asp classic no NET!
Usign LDAP or ADSI
i know:
mi server is server NT
name of domain=dip
name of group=my_group
name of user=my_user

With a input text box named "insert your userrname here"
and a post button "verify"

After pressign button, validate the user name if is present in the group, if YES goto my_ok_page.asp, if NO write an attention message in principal page.

Get the ip
do a lookup via nt-groups
get match of ip and username