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.