probability that two out of a group of 8 friends..

What is the probability that two out of a group of 8 friends will have the same birthday?

I know that you have to do 1- something, but I forget how to do these AT LEAST questions. help please?
thanks.

Read the material at this link.
http://mathforum.org/dr.math/faq/faq.birthdayprob.html