Well, in order to pay the cost of the mugic, you need to pay a certain number of mugic counters from a creature of the correct tribe (if the mugic belongs to a certain tribe, otherwise it can come from any creature). It doesn't matter which creature of yours pays the cost, just as long as they're from the same tribe as the mugic. This means that Creatures that aren't battling at the moment can still influence the fight.
In girum imus nocte et consumimur igni