A user can belong to multiple groups, but only one population. A population is a fundamental organizational unit, while groups offer more fine-grained control. For example, you could use a population to contain all your employees, and use a group to define subsets, such as Marketing, HR, Contractors, or US Employees.

A population-level group can contain users from that population only, but an environment-level group can contain users from different populations in the same environment.