MEMBERSHIP INTEREST PURCHASE AGREEMENT BY AND BETWEEN JAMES E. BAGENSTOSE, M.D., DAVID L. DAVIS, M.D., JOHN J. DUGGAN, M.D., JAMES A. O'NEILL, M.D., DAVID B. STEINER, M.D., ROGER L. TERRY, M.D., MICHAEL J. WIESER, M.D. MARK G. MCDONALD, M.D.