An apparatus (101) for a data communication system comprises a user group processor (207) which generates a user group for a first user. A set of content topics is determined and stored in a topic set storage (211). A communication processor (201) monitors a data communication of the user group and a content topic processor (205) determines the current content topic of the data communication by content data extraction. A notification processor (209) compares the current topic and the set of content topics and generates a notification signal in response to this comparison. The notification processor (209) may for example alert a user to a change of content topics for a trusted community defined by the user group.