My internet goes really slow when more than one person connects?

My internet goes really slow when more than one person connects?

I once was able to go on xbox live and youtube and my whole family would be connected and it would be super fast.
now it goes really slow if other people are connected.
(might be irrelevant but when we all connected we didnt use much internet, now we use heaps of internet up and so we have to watch our usage more)