1. A messaging system for transmitting messages between user terminals in a mobile telecommunications network, said system comprising: data storage means arranged to hold subscriber records, said subscriber records including a plurality of fields including a group field for storing data indicative of group membership of a subscriber, wherein said messaging system is arranged to: receive, over a data communications network, group membership data from a first subscriber; identify, using the group membership data, a remotely accessible information source corresponding to the group; request, over a data communications network, access to the identified remotely accessible information source on the basis of identification data associated with the group membership data, and, responsive to receipt of a response over the data communications network 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, the first storage action comprising updating the group field in the subscriber record so as to indicate authenticated membership of the group identified in the received group membership data; receive a message being transmitted between user terminals in the mobile telecommunications network, the message including transmission data identifying messaging parties, the messaging parties comprising a sending party and a receiving party, wherein at least one of the messaging parties comprises said first subscriber; identify said first subscriber as a messaging party; determine group membership of the first subscriber on the basis of the updated subscriber record; modify the message on the basis of the determined group membership; and transmit the message to the receiving party on the basis of the transmission data.