You can see who belongs to certain groups and send them messages or friend requests.
Click on Groups on the left part of the window (the sidebar). Then select a group in the main window.
Click the contextual menu button.
Choose Show Members.
The main window now displays people that are members of that group.
You can now select anyone in the main window and view their friends as well.
Some groups have privacy settings that disallow others from viewing their members, in this situation no one will be listed in the main window.