After the seven Dragon Balls have been earned by both Team of fighters during a given battle, Shenron can be summoned. Only one player can do this, however, and whoever does it first will reap the benefits (no matter who has earned more of the Dragon Balls. But there's another catch! If order to summon Shenron, you not only need all seven Dragon Balls but you will also need to have a Maximum Super Meter of Seven Complete Energy Bars! If you don't, you will not be able to summon the Great Dragon, no matter if all seven Dragon Balls have been obtained.
If you meet both requirements, you can summon Shenron by doing a Light Attack Auto Combo. After it appears, Shenron will grant you your choice of one of four different wishes: Restore My Health, Bring Back My Ally, Give Me The Ultimate Power, or Make Me Immortal. The Restore My Health Wish will see Shenron complete heal your character in play. The Bring Back My Alley Wish will revive one of your fallen teammates with 20% Health. The Give Me The Ultimate Power Wish will give your Team a second Sparkling Blast Icon, allowing you to use two in one match. And the Make Me Immortal Wish will see Shenron regenerate the Blue Health for your entire team AND auto-restore your Super Meter.
Even though the chances of actually obtaining all seven Dragon Balls and summon the Great Dragon Shenron are extremely slim, it can change the entire outcome of a match. But since it is so hard to do, it is always recommend to focus on winning and let the Dragon Balls and the number of which to fall as they may. If you get the chance, definitely go for it. If not, don't lose a match by trying to make it happen.