Layout menu contains item that let you choose from specific pre-created layouts of groups (one column, two columns, etc). The
Groups menu has items that work with groups directly (i.e. they modify the current layout by adding or creating groups, and the
Focus Group menu lets you change the keyboard focus between groups.
As such "Focus Group" it not a separate thing, it's just something that contains menu items that lets you focus a particular group of files.
You may want to investigate the Origami package if you haven't already. It makes creating groups, focusing them and moving files between them easier and more natural.