In the advanced search ( https://secure.phabricator.com/maniphest/query/advanced/ ) there is an "Assigned To" field and a check box "Show only unassigned tasks".

I could use searching only for assigned tasks. By having them grouped by assignee and restricted to a project representing a team, that would yield the work each team member is currently working on.

So I would either:

a) create mutually exclusive radio boxes:

( ) Show only unassigned tasks
( ) Show only assigned tasks

b) convert it to a selectable list

Show only [                     ]
           +-------------------+
           |  unassigned tasks |
           |  assigned tasks   |
           \-------------------/

I prefer radio boxes myself.