I am hosting an event. 80 invitations are sent out. How can I work out the probability of how many will actually attend the event?

You can't work this out without more information. You would need to know the probability of any one person accepting the invitation - suppose that value is p. Then you can determine the probability of k people out of 80 accepting by using the binomial probability formula:

n(S)= 80 invitations.
Let n(A) be the event that the members will attain.
Thus,
P(S) = n(A)/n(S)


the ratio between number of members attaining "A" to total number of members "S" will give you the Probability of members attaining...