  Communication rates over quantum channels can be boosted by entanglement, via
superadditivity phenomena or entanglement assistance. Superadditivity refers to
the capacity improvement from entangling inputs across multiple channel uses.
Nevertheless, when unlimited entanglement assistance is available, the
entanglement between channel uses becomes unnecessary -- the
entanglement-assisted (EA) capacity of a single-sender and single-receiver
channel is additive. We generalize the additivity of EA capacity to general
multiple-access channels (MACs) for the total communication rate. Furthermore,
for optical communication modelled as phase-insensitive bosonic Gaussian MACs,
we prove that the optimal total rate is achieved by Gaussian entanglement and
therefore can be efficiently evaluated. To benchmark entanglement's advantage,
we propose computable outer bounds for the capacity region without entanglement
assistance. Finally, we formulate an EA version of minimum entropy conjecture,
which leads to the additivity of the capacity region of phase-insensitive
bosonic Gaussian MACs if it is true. The computable limits confirm
entanglement's boosts in optical multiple-access communications.
