Same Birthday 

For a group size of n, the probability that all people have a different Birthday = $\frac{364}{365}\times\frac{363}{365}\times\frac{362}{365}...\frac{366n}{365}$ = p
The probability that at least one pair share a birthday = 1  p
Number of People  Chance of no pairs  Chance of at least one pair 
1  1  0 
2  0.997  0.003 
3  0.992  0.008 
4  0.984  0.016 
.  ...  ... 
22  0.524  0.476 
23  0.493  0.507 
If there are 23 or more (randomly chosen) people in a group then there is a more than even chance that two will share a birthday.