So 'joining' not the same as extended?
no
joining: express connects to the existing wifi, giving you access to the audio jack for airplay, and the USB port for printer sharing.
also if one of the N versions, will let you connect something to the ethernet jack that that will then be connected to your wireless network, useful for gaming consoles and such
extend: express connects to the existing wifi, and repeats it out. used for when you signal doesn't reach as far as you need it to.
also gives you access to airplay, printer, and lets you connect stuff to the ethernet port
when extending, you will see a drop in speed in the devices connected to the remote repeaters, as they have to use half the bandwidth to talk to your computer, and half to the main base.
because of the speed drop, if you don't need the range extension, don't use it.