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Set the uh-settings:for-groups:last-modified attribute to the current date and time, but truncate the seconds, and use this format: 20140315T1915 (March 15, 2014 7:15:59pm, note we truncate even if this is closer to 7:16pm. Please make sure your clock is synchronized with some time server).
Special considerations: who is performing the addition or deletion from the include and exclude groups? Is your application opting in or out on behalf of the user? If so, you should check whether the owner allowed that.