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.
The main privilege membership gets you are borrowing rights at the library. New library books are also what we use most of the income from membership fees on. Membership is also required to vote at the Annual General Meeting, at which the next year's committee is elected.
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, we'll bring the membership box so people can pay and be issued membership cards; if you want to join at another time, e-mail the committee and someone will bring the necessary materials to the next meeting.