I need guidance....   Can you point me in the right direction?  

I'm fairly weak at web design but I had someone design a simple web based (javascript, php) tool for computing and graphing business-specific calculations.
Currently, I have a few test users with the page protected using .htaccess.  Reception has been good and I want to be able to charge a small monthly or yearly subscription for using the tool.  I need some help understanding what remaining technologies or tools would be necessary in order for me to adequately limit access to the site to paying users.
Keep in mind that my number of users will be fairly low (200 or less) and that I would like to keep the costs low.

Primarily I want to avoid one user subscribing to the site and then sharing the credentials with his buddies at other companies.  
I don't expect you to give me a perfect solution, just point me in the right direction so that I know the right research to do.
 
I don't want to complicate the issue too much, but in case it helps here are my user types:.  
Manager Acct:)  There will be about 5 or 6 managers at a company sharing 1 or 2 computers.  They will subscribe to one version of the tool that allows them unlimited access (maybe from a particular computer or two).  I'm expecting this membership to cost more.
Regular Acct:)  There may also be regular employees that only use the tool 2-3 times per week on average but want to do it from multiple locations.  This membership would cost less.

Thanks again in advance!  
Chad

PS.  After doing a bit of research I found this link along with sample files.    http://www.plus2net.com/php_tutorial/php-membership.php    Perhaps this is a good start?