An adaptive control system automatically performs a loop over monitoring, assessment, and corrective action following a goal. A goal, for example, can be to keep the operational parameters of a controlled system within defined ranges. This work presents the way in which Hewlett-Packard's Utility Data Center (UDC) is extended by a control system for server group flexing for horizontally scalable applications. Capacity of server groups is automatically adjusted during operation by acquiring or releasing servers. In contrast to similar control systems that operate in clusters, the control system here operates in the heterogeneous environment of a commercial data center. Joining and releasing servers from its virtualized application environments is inherently more complex and requires securely crossing protection domains. The control system is based on the Grid's Web services standard OGSI leveraging OGSI's built-in security and event models. Adopting OGSI in the enterprise data center environment also introduces the opportunity for creating Grids in commercial enterprises.