How can i see players from different servers

I have seen this many times that i see players from different servers in my game how does this happens and how can i use this on my own servers

 

To be able to see players on other worlds do "/toggle ghosts none/low/medium/high/all" depending on how many ghosts you want to see. More ghosts = more lag, so keep that in mind. However if you want to do this on your own Minecraft server (not Wynncraft) then you would need to make your own plug-in for it. Maybe contact a Wynncraft developer to get an idea of how it is done, but they will probably not give you the code for it.