1. Apparatus for collecting subscription data for subscribers in a telecommunications system for use in metering usage of network resources, said apparatus comprising: data storage means operatively arranged to store subscriber records, said subscriber records including a plurality of fields including a group field for storing data indicative of group membership of a subscriber, said group field being initially empty; and data processing means operatively arranged to request access to a remotely accessible information source on the basis of identification data, and, responsive to receipt of a response from the remotely accessible information source, to perform a first storage action in respect of a response of a first type and to perform a second storage action in respect of a response of a second type, wherein, responsive to receipt of group membership data from the subscriber, the data processing means is operatively arranged to identify a remotely accessible information source corresponding to the group and to request access thereto on the basis of identification data associated with the received group membership data, and wherein, responsive to a response of the first type received from the remotely accessible information source, the data processing means is operatively arranged to update the group field in the subscriber record so as to indicate authenticated membership of the group identified in the received group membership data.