Settings

Choose: Cog → Manage apps → Settings (sidebar, Project Groups section)

Three settings available:

Enable Project Browser by GroupIf enabled, users will see a Browse By Group option in the Projects menu in the main horizontal navigation bar at the top of your screen.
Enable Project Browser by LeadIf enabled, users will see a Browse By Lead option in the Projects menu in the main horizontal navigation bar at the top of your screen.
Allow project administrators to associate projects with groupsIf enabled, then users with the Administer Projects permission will see the Group item in the menu on the project settings page and will be able to create or change the project’s association with the group. If disabled, the Group item will only be visible to users with JIRA Administrators or JIRA System Administrators global permission.