Multiple Monster Groups

Some of my custom Events (namely Monsters for Hire, Dwarfs, and Monster Bash, for example) result in more than one group of Monsters operating on the board at one time.  When this happens, each group of Monsters get their own Monster Phase.  To determine the order of each of the Monster Phases, roll 1D6 for each group and add the Initiative score of the Monsters (or lowest Initiative if there are multuple Monster types in one group), re-rolling ties.