What you want would require, at least, the creation of 3 ACL's. One would make it mandatory for a user to authenticate:
"acl password proxy_auth REQUIRED"
The other would define a group or class of users which will be allowed to access to some specific ports:
"acl good_guys proxy_auth username1 username2 etc"
The third will the set of ports you want to give access to:
"acl nice_port port 6666 7777 8888"
After that, you define an http_access line that will combine those 3 ACL's:
"http_access allow password good_guys nice_port"
This line means: "Allow access throught this proxy, to the ports specified in the nice_port ACL, to all the authenticated users who are part of the good_guys ACL".
Any other users, although authenticated, would not be allowed to connect to those example ports...
Hope that helps...