World Conventions, in order to increase funds, have traditionally sold two classes of membership: attending and supporting. Supporting Members receive (theoretically) all publications of the convention, and may vote by mail for awards and future convention sites; Attending Members have all the rights of Supporting Members, plus the right to attend the convention. Supporting Memberships may be converted to Attending Memberships by payment of an additional fee. On the grounds that "Supporting" memberships didn't actually support the convention (some cons had priced Supporting Memberships so low that they didn't even pay for the publications they received), and were only a way of temporizing in case someone decided months later to actually attend, L.A.con II, the 1984 Worldcon, sold Corresponding Memberships, instead. These had the right of receiving publications and of voting, but not of being converted to Attending Memberships.
Some groups bidding for the Worldcon have sold Pre-Supporting Memberships (or, occasionally, Pre-Opposing Memberships, at a higher fee), purchasers of which received rebates or reduction of their membership fees if the group won. With the escalation of the size of the Worldcon, however, the number of people who can (and will) misconstrue rights and privileges of their membership type has also escalated, causing some bidding groups to avoid such discounts in actual membership rates. (Thus, L.A.con II, instead of Pre-supporting Memberships, sold memberships in a separate group, Friends of L.A. in '84, to support the bid. Friends (and Enemies, the equivalent of Pre-Opposing) got special privileges, buttons, etc., but paid the full membership rate.)
Membership fees were originally almost insignificant. The $1.00 fee of the 40's increased to $2.00 in the mid-50's, $3.00 in the mid-60's, $6.00 by 1970. Conventions began a series of time-dependent fee increases that could range from $20 at the cheapest time to $75 at the door of the convention (rates for the 1984 Worldcon). Many fans found that it was safer to buy a membership at the cheap rate if there was even the slightest possibility of their attending the convention. They could also buy a "Guest of…" membership at that rate just in case they found someone to invite to the con later. If one or both memberships weren't needed, they could probably be sold to latecomers from the local fandom group at a bargain to both purchaser and seller.