You need the Delpinus and at least 5 crew members to get these specials. Get your SP (spirit points) in battle to full and a crew SP icon will appear next to the item. This enables all of your active crew members to attack. Repeat these steps to get another special. You need all 4 people alive for this. This one combines the strength of all 4 people, and is the strongest special in the game.

When Vyse has his own ship, he is able to look for 22 crew members but you need at least 5 to do this. When you get 48 spirit points in battle (not ship battle), you can choose to use the Crew Special. It is the strongest in the game, depending what crew you have active -- they can heal you as well as attack the opponent.