  In this paper we analyze a constant multiple-input multiple-output
interference channel where a set of active users are cooperating through
interference alignment while a set of secondary users desire access to the
channel. We derive the minimum number of secondary transmit antennas required
so that a secondary user can use the channel without affecting the sum rate of
the active users, under a zero-forcing equalization assumption. When the
secondary users have enough antennas, we derive several secondary user
precoders that approximately maximize the secondary users' sum rate without
changing the sum rate of the active users. When the secondary users do not have
enough antennas, we perform numerical optimization to find secondary user
precoders that cause minimum degradation to the sum rate of the active users.
Through simulations, we confirm that i) with enough antennas at the secondary
users, gains equivalent to the case of all the users cooperating through
interference alignment is obtainable, and ii) when the secondary users do not
have enough antennas, large rate losses at the active users can be avoided.
