Groups
define an ordered collection of processes, each with a rank, and it is this
group that defines the low-level names for inter-process communication (ranks
are used for sending and receiving). Thus, groups define a scope for process
names in point-to-point communication. In addition, groups define the scope
of collective operations. Groups may be manipulated separately from
communicators in MPI , but only communicators can be used in
communication operations.
MPI-Standard for MARMOT