It doesn't matter whether you all use the same frequency or not, there is more than enough wireless bandwidth for 5 simultaneous users. You will all have to share the broadband bandwidth which will be much lower anyway.
are you selecting the wifi channels manually or using automatic?
you can try downloading inssider3 (istumbler if MAC) and then run it. This will show the broadcasting networks round about you and their channels including your own. If you then enter your router and change your wireless channel to a free or less congested channel.