Go here, http://www.triathlon.org/rankings/index.htm, then at the bottom of the page click on World Ranking Points System.

For certain races (WC?), you have to be invited.

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clm
Nashville, TN
https://twitter.com/ironclm | http://ironclm.typepad.com

to add to cathy's information, anyone with a pro license (or not if you're in europe) can participate in an "international points race", provided that your equipment conforms to ITU draft-legal rules (no disks, shortie aerobars). world cup entry is very limited (and thus pretty difficult unless you're willing to go to some far-away place where no one else wants to race), is based on current ITU rank, and is done through your NGB (in the US of course that's USAT).