When they visit O'Brien, what does he tell them about the Brotherhood?

In Part 2, Chapter 8, Julia and Winston go to visit
O'Brien to learn more about the Brotherhood.  They are finally going to actually do
something to oppose the Party.

At O'Brien's apartment, they
are told many things about the Brotherhood so I am not really sure which thing you are
asking about.

I think that the two most important things
are these.  First, they are told that the Brotherhood exists and that Goldstein is its
leader.  Second, they are told that the group is not really as they might imagine it. 
It is not some big organization where all the members can know one another and act
together.  Instead, it is very shadowy and no one really knows who else is in it.  It is
pretty much a hopeless thing because there is no one to support a member and the members
are all going to get caught and killed.

So they learn that
the Brotherhood exists but that it is a pretty suicidal thing to
join.