  Due to the potentially large number of units involved, the interaction with a
multi-robot system is likely to exceed the limits of the span of apprehension
of any individual human operator. In previous work, we studied how this issue
can be tackled by interacting with the robots in two modalities --
environment-oriented and robot-oriented. In this paper, we study how this
concept can be applied to the case in which multiple human operators perform
supervisory control on a multi-robot system. While the presence of extra
operators suggests that more complex tasks could be accomplished, little
research exists on how this could be achieved efficiently. In particular, one
challenge arises -- the out-of-the-loop performance problem caused by a lack of
engagement in the task, awareness of its state, and trust in the system and in
the other operators. Through a user study involving 28 human operators and 8
real robots, we study how the concept of mixed granularity in multi-human
multi-robot interaction affects user engagement, awareness, and trust while
balancing the workload between multiple operators.
