In addition to what tiny said:
Run this command on your current master toon.
Run this command on each toon which will be a 'slave' or a 'follower'.
When you switch masters, because the master died, or for any other reason. You will want to issue the /of master command on the new master, and then the /of slave command on all slaves.
An example would be to create 2 macros on every toon. One containing /of slave, and the other containing /of master.
Fire the /of master macro on the toon you just switched to, and then fire the /of slave macro on the other toons.
Thirdly, you will want a /of follow macro. After firing the master and slave macros, fire this /of follow macro on your slaves and they should start following the master.
How you fire or trigger the macros will depend on which multiboxing software you are using, but it should be quite simple to set it up.