Sadly, reality is not like the movies. Defensive strategies can be entirely specific to the scenario, circumstances making these scenarios far more difficult. What you need to think first and most importantly is: go home safely.
How to defeat multiple opponents by using Jiu-Jitsu: focus on staying safe, using the three main principals: diffuse, evade, attack.
First of all, you need to diffuse the combative energy. This energy is often caused by ego or alcohol. Taking things personally causes a battle of egos, which inevitably leads to a fight. It jeopardizes your true sense of self, leaving it in the hands of others. Avoid this. It’s about them, not you. The bottom line: be self aware. Bodily movements contribute to the combative energy threshold, which brings a person closer to confrontation. With more people, there are more contributions to this threshold. Communication is 80% non verbal. Winning is defeating an external opponent. Leaving the scene safely is far more rewarding, though a great deal more difficult.
Failing diffusion, evading the attack is the next principal. Evasion is possible by utilizing effective distance management. Make yourself unreachable. Run, turn a corner, get to a more populated area. Sometimes, to reach this freedom, you may need to throw a hit. This is acceptable if it would allow you to create the avenue of escape. Otherwise, you are merely escalating the situation.
Finally, and only if it is solely to protect people and not egos, attack. Attack first and attack fast. Not everybody has great striking ability though. Knocking a person out is extremely difficult. A recommended method is to hit the opponent’s chin with your elbow. If possible, following this, return to the previous principals.If they are still impossible, you need to take a second shot. This is not easy. The enemy will want to grab and control you. Expect, accept and avoid it. Disengage or strike. If not possible, take-down and break. Get back to your feet quickly.
To sum it up, how to defeat multiple opponents by using Jiu-Jitsu: even if someone initiates the physical attack, you can diffuse the situation by understanding your sense of self.