Express or any apple router is not really good product for this task.
As it stands.. you have only ONE option.. wired connection between your room and the main router. Either ethernet which is best by far or using powerline adapters.
The poorer alternative is to use Express as a wireless bridge.. and add another router. If you want to do this on the cheap.. a second hand Airport Extreme Gen5 or Gen6 .. although IMHO the Gen6 while having much better wireless is also not as hardy and reliable as the older model Gen5 even if it is now 5 years old and counting.
Of course if you don't need apple router then the world is a much broader place.
For your friends not to see your devices (and visa versa??).. the trick would be for you to bridge to guest wireless on the Century Link modem router.. or if you wire things.. to use vlan on the Century Link modem router.. which may or maynot exist.
You will have to double NAT.. unless the CenturyLink modem router is a lot more flexible than most I have seen.. this is not a major problem except for gaming.. in which case you can run your game console.. whatever.. directly from the Century Link unit.
Perhaps tell us exactly make and model of this CenturyLink box?? We will have a better idea of what is possible and what is not.
