It won’t let my friends join my world

So me and me and my friends try to play in my world but some of them can’t and some of them can and the people that say they can’t join they say the loading background will freeze up and they can’t join the world

they tired to unplug the systems 

turning off the routers and everything else 

We tried to reload the world but it won’t work at all the two same people can’t join the game