Assuming you're playing a Generation VI game, what you'll do is access the PSS (it's the yellow menu that can appear on the bottom screen outside of battle). You'll click on their character when it appears and the rest shouldn't be hard to figure out.
CJ
You not gon' ge me!
Towns Folk
#3
when you register someone with a gen vi game, switch on the game. In your PSS (Player Searsh System), the first list is of friends. It will show their picture even if they are offline. Select the player and select battle.
