The Dynamic group is based off a set of rules you detemine that will populate the Dynamic Group's membership. In behind the scenes there are two scheduled tasks that run to ensure that the memberships are current and accurate:
1) Dynamic Group Checker:
Verifies configuration of Dynamic Groups to ensure that the membership list of each Dynamic Group is updated in a timely manner. If update does not occur within a certain time period, this task adjusts the edsaDGOriginatingService setting that the Dynamic Group Updater task relies upon. The time period is specified on the Parameters tab in the Task Properties dialog box, and can be configured by assigning an appropriate value to the 'DG update latency threshold' parameter.
2) Dynamic Group Updater:
Rebuilds membership lists of Dynamic Groups in accordance with the current membership rule settings. Processes only those Dynamic Groups that have edsaDGOriginatingService set to the Administration Service on which this task is executed. The edsaDGOriginatingService setting can be configured on the Membership Rules tab in the Dynamic Group Properties dialog box.
For more infromation on Dynamic Group rules and configuration options, please see Page 444 of the Admin Guide.