MPI applications may start new processes through an interface to an external process manager.
MPI_COMM_SPAWN starts MPI processes and establishes communication with them, returning an intercommunicator. MPI_COMM_SPAWN_MULTIPLE starts several different binaries (or the same binary with different arguments), placing them in the same MPI_COMM_WORLD and returning an intercommunicator.
MPI uses the group abstraction to represent processes. A process is identified by a (group, rank) pair.