The following is a good way to verify whether your squid configuration with NTLM authentication is properly set up to utilize Windows Active Directory group memberships:

# echo “username windowsgroup” | /usr/lib/squid/wbinfo_group.pl -d
Debugging mode ON.
Got username windowsgroup from squid
User: –username–
Group: –windowsgroup–
SID: -S-1-5-21-915438365-207112795-1232828436-3341-
GID: -16777237-
Sending OK to squid
OK