The Mixer groups feature is only available when Show Advanced Tools is selected in the Advanced preferences pane.
Prior to mixing, you may find it useful to define some logical channel strip groups. You could, for example, group all drum channel strips under one drum group. This would allow you to control the group meters (volume, pan, mute, solo, sends, and so on) using a single control, while still maintaining the relative parameter values of each channel strip. You can have a maximum of 32 groups, and a channel strip can be a member of multiple groups.
You can also use aux channel strips for grouping purposes, but the emphasis is on controlling channel strip signal flow.