When you have users (staff, employees, members et al.) who must apply, join, or request to be a member of your group, it's very likely you want to collect information on them so you can review their application, determine if they are an employee, screen them etc.,
In order to activate this, navigate to "Customize", select "Users" and you will see a few options as shown below.
Membership Requirements: enables you to turn on the ability to block users from joining until they have you review their application and agree.
Membership Request Fields: is similar to all the custom field generation pages and lets you define the fields for users to fill in before they can join your group.
Administrative fields (a platinum option): enables you to customize fields to collect about your users beyond the basic fields we have as a default.