Ardour has always included edit and mix groups, similar to those found in ProTools (and elsewhere). In release 3.0, we have unified and extended the basic ideas of grouping into a more flexible system.
Rather than use separate edit/mix groups, 3.0 now features groups whose properties are entirely configurable by the user. These properties can be seen in the edit dialog and the group tab of the editor window behind it:
Once a group has been defined, tracks and/or busses can be added to it as normal (via the group or "g" button). Group tabs to the left of the editor window and above the mixer strips in the mixer window can then be used to carry out various operations on the group:
Perhaps the most exciting of the new actions is "Subgroup", which will, in a single step, create a new bus for the entire group, and route the outputs of each member of the group to the bus. In a single click you can also fit all members of the group into the edit window ("Fit to Window"), or just bring them together in the editor window without adjusting their sizes ("Collect").