David Pulver wrote up "Tactical Mass Combat" in
Pyramid #3/44: Alternate GURPS II.. If I were you I'd start there. That's what I use when I need to go for large combats, but don't want to break out Mass Combat completely. It works surprisingly well.
That said if you don't like that, the easiest way to know do this sort of thing is to prepare everything you can in advance. So if each PC has five or six characters then they need to know what those characters do. Also, lots and lots of preparated actions "Alpha, Bravo, and Charlie aim. Delta and Echo lay down cover fire." That's really it - lots of prep and you can pull it off pretty easily. One thing I also did was use a single roll for attacks (average skill of all involved) and check the margin. Each point of success means another attack landed beyond the first. So if you have five guys in the squad and all are attacking then you'd need to succeed by 4 or more for all to hit. Do the same for Active Defenses and things should go pretty quickly.