How do you combine Minion groups?

I usually have one group spend a Maneuver to form up with another group, depending on their relative distance from each other. Simple enough?

I have done it this way, but I think you might also read this as an Action, along the lines of "Instructing allies with a series of orders." (No skill check needed, obviously.)

I'd say an action is too strict. A manoeuvre is fine, with breaking up a group as an incidental.

Yeah - a maneuver at most, an incidental seems fine. I might take the quality of the troops into account, too. A gang of Gamorreans might take an action to combine, but a two squads of Stormtroopers that come from the same unit and have worked together would be an incidental.

Since minion groups are more a convenience factor for the GM, breaking them up or merging them isn't really any action (Action, Maneuver, Incidental) on their part. They can continue to shoot wildly at all the various players (and just happen to hit one specific one) even if they are suddenly part of a group that they weren't part of a moment ago, so long as the minions only partake in one group per round of combat.

The GM kit's squad rules has minions able to rejoin a formed Squad as two maneuvers. While slightly different in that there's no squad or squad leader, the rest is close enough that I'd probably do the same here.