In this scenario, the unrestricted group is the population of individuals who are in the selection process. For example, Held and Foley (1994, Table 3) https://conservancy.umn.edu/bitstream/handle/11299/116989/v18n4p355.pdf shows the consequences of different selection ratios on validity coefficient estimations.
To summarize, the unrestricted group is the entire pool of applicants who take your test. The restricted group is the ones who are selected based on your selection criteria.
Any cognitive ability test should have a manual, and that manual should contain validity information, including SD of the population. Many of them are normed in such a way as to make it simple (e.g., the SD of WAIS is 15, the SD of Stanford-Binet is 10). Obtain and use the values in the test manual.
However, I don't think this should be done in practice. The population of concern here isn't all working-age adults in the world/region, the population is all working-age adults in the world/region who have applied to work at the organization.