Groups allow you to organize your teams by departments, squads, functions, skills and whatever else you see fit. Once a group is created and members have been added to it, you can assign groups to cards and mention them just like with any other member. You can also share collections to groups and filter by them in the member criteria in reports.
Groups are available from the de-bundled apps menu.
Once in the de-bundled app, you can create a group,
and then add members and set their permissions (members are included in actions taken affecting this group and owners have full editing rights to the group).
In the details pane, you can change the group's name and add a logo or delete the group.