Membership of the society is priced at 3n + 1 pounds for n years - for the mathematically uninclined, that's 4, 7, and 10 pounds for 1, 2, and 3 years, respectively, although membership can be purchased for as many years as you like at a time.
Who can buy membership? Anyone! There are no rules on University affiliation or age for becoming a CUSFS member or indeed coming to CUSFS events.
The main privilege membership gets you are borrowing rights at the library. Membership is also required to vote or stand for election at the Annual General Meeting, at which the committee for the following year is elected. CUSFS members also get 10% off at the Cambridge branch of Forbidden Planet upon presentation of your membership card - it's far and away the best place locally to buy comics, books, DVDs, memorabilia and so on. Membership may also provide a slight discount for the occasional CUSFS ticketed event, such as our annual picnic with other geeky societies.
Other than these, the society's policy on membership is very lax. You are free to attend our events without being a member, and you can buy membership at any point. For the first few meetings of every academic year and at the AGM, we'll bring the membership box so people can pay and be issued membership cards; if you want to join at another time, let the memsec know and someone will bring the necessary materials to the next meeting. If you have paid for membership and haven't yet received a card, please get in touch.
Lastly, unicorns can be acquired through the donation of a certain number of books, typically around 4 - 6. If you want to pay in this way, the librarian can sort it out. Membership can in general sadly no longer be acquired with books.