The problem of wireless spectrum sharing is investigated using a multi-server queueing system that models a group of channels and a multiplexed arrival process formed by a primary and secondary group of users. Motivated by under-utilized capacity in RF spectrum owned by the primary group, this work examines the impact of secondary user access patterns on blocking probabilities and achievable improvement in spectrum utilization with statistical multiplexing. The analytical framework presented allows estimation of the blocking for any renewal process characterization of secondary access times. In this work, Gamma distributed inter-access times are chosen to examine the impact of their index of dispersion (IDI) on the excess blocking probabilities of the Poisson distributed primary group. It is shown that by reducing the IDI value to be below unity, one can increase the secondary system utilization in the shared spectrum with minimal impact on the primary system. Combined with an increase in the number of excess channels available for secondary use, a feasible spectrum sharing paradigm can be proposed by shaping the access patterns of secondary systems.