Ok so the command is /tellraw @a {text:"Moo_Cow joined the game."color:yellow}, but this can be anything! All you need to do is change what is says and/or color! And you dont need a command block! Now lets be real you can use "Herobrine joined the game", but thats old now.So use whatever you want but you should probly cheak and make sure its working and the color is right. Have fun!!!!!