The Guest-to-Member Ratio is very simply the number of Guests who visit a Club during a particular time period, divided by the number who become Members. Not all Guests become Members. Not all Guests are Prospects.
Clubs generally report that 20% to 50% of Guests become Members, if those Guests are also Prospects. Therefore, if a Club has 15 Members and wishes to get to 20 Members, it needs five additional Members but more Guests than that. If the Guest-to-Member Ratio is 33%, then the Club needs 15 Guests in order to yield five Members from its Public Relations efforts.