Many years ago I had a program that suited just your needs. Each user had a username/pass into the program which would popup just when you dialled up.
You had to logon to the program to then access the internet, and it kept a log of time and costs etc.
Problem is I can't remember it's name at all, too long ago.
Check if your ISP offers an 'Anytime' deal - so for a set price per month you can dial up for free.
[Edit]
I think
www.internet-timer.co.uk was the program I am thinking about.