The FIM Reporting service automatically installs management packs to create a number of reports. At http://technet.microsoft.com/en-us/library/jj133843, the default reports are described, but I have the information copied here for your convenience:
Group Membership Change report: This report provides key information about group membership modifications in FIM, including the user account that approved the group change, the type of change, and any related requests or policy rules related to this change.
Set Membership Change report: This report provides key information about set membership modifications in FIM, including account information about the user who joined or left a set, approvers (if any), and any related requests or policy rules related to this change.
Group History report: This report provides information about changes to key attributes on group resources in FIM, including group filter, owner, type, domain, and membership.
