A switch module is designed to facilitate user definition of a group of switch modules that control the same lamp. A lamp unit (12) containing the lamp responds selectively to messages from the switch modules (10, 10′) when the messages comprise ID information matching with ID information stored in the lamp unit (12). A group is formed by detecting user control operation of sensors (100) in the switch modules (10, 10′), transmitting messages from the switch modules (10, 10′) in response and determining from the messages whether the switch modules (10′) have both been operated within a time interval with a duration of less than a predetermined threshold. If so, the ID information value in memories (104) of the switch module (10) and/or the further switch module (10′) are set to matching ID values, for use in the messages to control the lamp unit.