[MPICH] How to change the communication groups

[Kevin C.]

I've been looking and looking on the internet for documentation on 
programming in MPI but couldn't find any.

What am needing to know is how to change the communication group for 
MPICH-1.2.1. Basically so that when the master process 0 broadcasts a 
message it will only be heard by the processes in a specific communication 
group.

Many thanks
Kev

PS: Did anyone find out how to dynamically create/destroy MPI processes 
on-the-fly / in-real-time for MPICH-2?