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to expire a user from a group, sorry i wasn't more clear, at work so i'm in a hurry
ex: nukescripts.net has a paypal block which is made to work with their NSN Groups system. you define the group that the user would join for making a paypal payment, define how long they will be part of the group, etc
so i could say that when a user makes a $5 payment, they would be entered into a group that would give them access to a pay-only area of the site, and they would be kept in that group for a week or a month or a year, however long i want. i believe the amount can be specified down to the number of days.
if you want to see the NSN Groups code, check out
http://www.nukescripts.net (nsn groups for php-nuke) or check out
http://www.opensitemanager.org for the OSM CMS (has nsn groups fully integrated)
if you need more code examples, let me know, i have code on my *nuke sites that might help you out.