Groups are a collection of users.  Groups can also have administrator(s) (or a lead) who can assign the group's members to events.  If you would like to allow volunteers to register their group, you can create a form.  To do this, go to Forms and Manage.  Then click on the elipsis, and Generate Group Registration.  Enter the form name and minimum/maximum members to be added.  You can then send your prospective volunteer leads to this form.  There they will fill out the form with a team name, members and any other fields you wish to add.  As a administrator in Duplie, you can then go into the submitted form and Create Group.  The submitter of the form will be created as the group administrator.  Other team member names will be created within the group.

As a administrator in Duplie, you can create a team/group manually by adding a group.  Then add an admin.  Make sure the admin is allowed to manage event roles for all group members.  Then you can add the group users.

By having teams/groups with admins, you now have the ability to invite the group admins to an event.  The admins can then sign up their users for the event.  To invite a group admin, go into the event and click the "cog" icon and Invite.  Then select Group admins.  The group admin will recieve an invite email which will automatically log them in to sign up their group's members for the event.