Groups are easy way to organize large amount of contacts.

To create a Group press Add Group button in the left-side bar:

In a pop-up Group Info dialog you can specify Group info and add Group members from all contact list using Group members tab.

Groups could be arranged in tree structure. To make a sub-group just drag needed group to parent group with left mouse button.

Each contact could be a member of several groups. You can easily move contacts from one group to another  dragging them with left mouse button. You can also edit each contact membership in special Membership field:

It could be found at the bottom of Contacts screen or in Contact Info dialog. By pressing "..." button there will be pop-up dialog where you can enable needed groups:

Groups could automatically arranged by Name or Last Modified date: