In socket.io v.0.9.x you could access al connected clients by using the aptly named function clients:

io.sockets.clients().map(function(socket) {
    ...
})

In v.1.x that does not work. One one to do it would be to access the underlying engine.io:

To get an array of connected clients:

io.sockets.sockets.map(function(socket) {
    ...
})

To get an array of the connected client ids:

//Get array of the connected clients
Object.keys(io.eio.clients)    //=> [ 'US8AxrUrrDF_G7ZUAAAA', 'Ov2Ca24Olkhf2NHbAAAB' ]

To get a count of connected clients:

io.eio.clientsCount

NOTE: You could also use the io.engine property.