A goal of this society is to acknowledge you are the most intelligent person ever on Earth. The word "Grail" symbolizes the concept of g or "general intelligence", defined as the common factor in mental ability test score variance, personified in you. In the process of qualifying, you will contribute to the higher goal of enabling the norming of the very highest score levels on I.Q. tests for the high range.
Qualification
Since it is estimated that roughly a hundred billion of the species Homo sapiens have lived until now, the ideal admission level is an I.Q. test score reached by one in a hundred billion persons, which theoretically takes place about 6.7 standard deviations above the mean, so around I.Q. 200 when the standard deviation (σ) is set at 15.
For several reasons this value can not be correct; for instance, the mean and σ of the group of all humans that have lived will not be the same as those of the current adult Western population to which our norms refer; projected onto today's I.Q. scale, the level of 1 in 100 billion Homo sapiens is logically somewhat below 200 but higher than 190. And a normal distribution of that total group may be incompatible with normal distributions with different means and σs of the subgroups that comprise it, such as sexes, historical eras, subspecies, or peoples (although on the other hand, amalgams of normal distributions do tend toward normality themselves). And an actual distribution is never exactly "normal", let alone at such an extreme distance from the mean (actually it can on good grounds only be expected to approximate normality within plus or minus about 2 σ from the mean). So that is why the admission level is called "ideal".
For reasons of simplicity and symbolism though, a straightforward ".99999999999" or "1 - 10-11" will suffice to become the Grail Society's member. Qualifying tests: all of the tests by Paul Cooijmans with norms at or above that level, as well as a score on the (free of charge) Test for ex