asterisk users - Jan 2013 - Question on Confbridge menu item dialplan_exec

I like the example of using that to add somebody to the conference, but
what I don't see is how the dialplan can know what conference the menu
item was called from.  I was hoping that some variable might have been set,
but don't see it in the sources.  Is the idea to do that outside of the
call to Confbridge?