This plugin requires Citizens and it allows you to create NPCs that run commands when you right-click them.
You can set your own command in order to move the player to another server/ world or you could open
a server selector gui. The plugin can display the amount of players from one or multiple worlds or servers.
It can display a sum of players like this: arena1+arena2+arena3.
Q: Why /css join doesn't show up?
A: You need to set bungeecord: true in spigot.yml in order to have it registered.