You can sort of do groups but not utilising PAM as far as I know.
Create a file with all the usernames in the group you want (one per line)
Assuming the above is saved in the file /etc/squid/group1
then change your acl entry to this.
acl anyaccess proxy_auth "/etc/squid/group1"
http_access allow blacklist anyaccess
The ACL now collects the usernames from the file rather than just ones you specify in the config file.