Thats quite easy 
When you have a drink with on or more people, just ask them for a coin, but don´t tell them the reason.
The value is not important. One Cent is enough, two Euro is also god (for you).
When you collected the money, put it in YOUR pocked and very importend you are NOT ALLOWED to give it back !
Now, give the new member(s) a very warm wellcome and tell them this short story:
"Wellcome!
With this coin, you have bought a lifetime 
membership for the honorable Buffalo-Club-International."
The Buffalo Club was foundet in the 1920ies in South Afrika and become over the centuries a world-wide cross culture organisation.
There are only two rulez in this honorable club.
1.) From now on and for the rest of your life, you have to drink every alcohol with your left hand.
2.) When another honorable Buffalo Club member spot you drinking alcohol with your right hand, he is forced to shout you a BUFFALO
In this case, you have to drink your alcoholic drink at once.
(under the other guy´s loud shouting of "Buffalo, Buffalo, Buffalo...")
If you have been called a Buffalo by drinking a non-alcholic drink - or the other guy was simply wrong (left and right becomes more difficult 
related to your own count of drinks) THANNNN the guy who was wrong to shout you a Buffalo have to drink his own drink at once.
TWO RULEZ - for the rest of your life - You will have fun!
)