Instructions
Enter a level from The Nexus. You can join another player's game from any level in the game.
Press "Start" to access the menu.
Select the "Inventory" tab.
Use the directional pad to highlight either the Blue Eye Stone, if you wish to join someone's game to help her, or the Black Eye Stone, if you wish to join someone's game as an invader.
Press "X" to use the stone. If using the Blue Eye Stone, blue symbols will appear on the ground in the other player's game, and you must wait for the player to summon you. If using the Black Eye Stone, you will automatically connect to someone else's game as an invader, attempting to defeat him.