Look at the whole group--say, 5 soldiers doing the same Shooting action--as a single Wild Card character. So they roll one Shooting Die and one Wild Die. The idea is that they're working as a team, so they have a better chance of success. However, since they're rolling a Wild Die, they also have the possibility of a Critical Failure.
- Note: In the event the extras' don't have the same die type for the skill used, choose the lowest applicable die. For example if four extras had a d8 shooting and one had a d10 shooting, the whole group would roll one shooting die at d8.
When the group is down one, two or three guys, they are considered "Wounded" with the appropriate penalty applied to their future actions. Upon reaching "Incapacitated," there's just one left, so he's a normal Extra - who should probably be dubbed Lucky Eddie. (Note that since Followers are replaced when it would be reasonable to do so, unless you're actually treating them in a particularly callous manner, this isn't permanently gimping the Edge. It just means there's a period where they are less effective than usual.)
Alternately, you can just have five extras all rolling independently. In that case, there's no Wild Die, just the d6 rolled to check for Critical Failures if the first die is a 1.
For Quick Encounters and non-combat tasks, use the Group Die method outlined above. In actual, tactical combat use the individual method with a single wild die to check for critical failures.