PIDF only defines tuples for one presentity.  In many cases, it is
useful to allow presentities to refer to groups of other presentities. 
For example, a presentity \verb|all@example.com| might consist of
\begin{flushleft}
\verb|marketing@example.com|,\\
\verb|engineering@example.com|,
\verb|finance@example.com|.
\end{flushleft}
\verb|engineering@example.com| might in
turn have presentities 
\begin{flushleft}
\verb|alice@example.com|,\\
\verb|bob@example.org| (an intern),
\verb|carol@example.com|
\end{flushleft}

\section{Groups of Presentities}
\label{sec:groups}

In many practical applications, a watcher wants to subscribe to groups
of presentities rather than individuals. For example, the group
membership may change over time and it may thus be difficult to
subscribe to all members. If the group is large, the effort of
subscription and their renewals may add significant burden to the
watcher.

There are several different approaches to group subscriptions:

\begin{description}
\item[Group only:] The watcher subscribes to a group and only cares
about the status of the group as a whole.  There is no protocol
difference to subscribing to an individual and thus no need for
extensions.

\item[Subscription only:] The watcher subscribes to a group, but
receives individual notifications. This does not require an extensions
to PIDF. Details are in \citeinform{draft-ietf-simple-event-list}.

\item[Subscription with redirection:] The watcher subscribes to a group.
The presence document identifies the group members and allows the
watcher to subscribe to each member individually. In PIDF, this is
expressed by a ``pres'' URI in the \xmlelement{contact} element. Each
such presentity can in turn be a group, recursively. TBD: How does the
watcher find out if group membership has changed? We don't want to list
all members in each PIDF notification. This basically becomes
draft-roach-sip-list-template.

\item[Subscription with full status:] A single notification contains
tuples from all presentities that have changed status since the last
notification.  We allow ``recursive'' presence definitions, where a
\xmlelement{presence} element contains other \xmlelement{presence}
elements, encapsulated as \xmlelement{members}
(Section~\ref{sec:members}).

\end{description}
