1. A method of advertising to a remote unit belonging to a user group of remote units, the method comprising: identifying, by the remote unit, a context associated with a communication sent to or received by the remote unit belonging to the user group of remote units; determining, on a server, whether the identified context is associated with one or more advertisements by querying correlation data to identify advertisements having context that correlates to the identified context; and when the identified context is associated with one or more advertisements, transmitting, by the server, the one or more advertisements to at least one remote unit belonging to the user group wherein transmitting the one or more advertisements comprises transmitting the one or more advertisements to at least one remote unit belonging to the user group that did not send or receive a communication containing the identified context.
2. The method of claim 1 wherein before determining whether the identified context is associated with one or more advertisements, the method comprises: determining, by the server, whether the identified context is contained in a context index; when the identified context is not contained in the context index, determining, by the server, whether the identified context is contained in a candidate list; and when the identified context is not contained in the candidate list, adding, by the server, the identified context to the candidate list.