The University societies' fair is held at Kelsey Kerridge (a sports centre in town). There'll be regular trips there, so you don't need to worry about finding it on your own. It's lots of fun and totally manic, you'll sign up for loads of things you have no intention of ever pursuing, but that's what happens when they bribe you with chocolate! Go for it, though- you can always ask to be removed from the email lists later. It's the ideal time to start something completely new, or to keep something going. Once again, an ideal chance to meet like-minded people from the whole University.
Here at Newnham we'll also have our own societies' fair which will take place on Saturday 9th October in the afternoon. Joining a College society is not only a great way to meet people from College but also they often require very low commitment so there's no reason why you can't have a go at several different activities. There is a list of the societies available at Newnham in the Freshers' Handbook that you will receive after Results Day. If none of these societies appeal to you, there is also the opportunity to start your own!