In this paper, the multinode amplify-and-forward cooperative communications for a network of N nodes is studied via the novel concept of many-to-many space-time network coding (M2M-STNC). Communication under the M2M-STNC scheme is performed over two phases: (1) the broadcasting phase and (2) the cooperation phase. In the former phase, each node broadcasts its data symbol to all the other nodes in the network in its allocated time slot, while in the latter phase, simultaneous transmissions from N-1 nodes to a destination node take place in their time slot. In addition, the M2M-STNC scheme with optimal node selection (i.e., M2M-STNC-ONS) is proposed. In this scheme, the optimal relay node is selected based on the maximum harmonic mean value of the source, intermediate, and destination nodes’ scaled instantaneous channel gains. Theoretical symbol-error-rate analysis for M-ary phase shift keying (M-PSK) modulation is derived for both the M2M-STNC and M2M-STNC-ONS schemes. Also, the effect of timing synchronization errors and imperfect channel state information on the SER performance and achievable rates is analytically studied. It is shown that the proposed M2M-STNC-ONS scheme outperforms the M2M-STNC scheme and is less sensitive to timing offsets and channel estimation errors. It is envisioned that the M2M-STNC-ONS scheme will serve as a potential many-to-many cooperative communication scheme with applications spanning sensor and mobile ad hoc networks.