My understanding is that the limit was 63 groups per side. I suppose that, depending on what other groups on that side are doing, you may not be able to place as many individuals as you want.
Assuming it's a very simple set-up: American cowboys riding into a Russian-held town -
set the Resistance side to be friendly to East
place a Resistance Officer with a high rank near each individual and set his "probability of presence" to zero
press F2 'Groups'
left-click-and-hold on the individual, dragging a line to the non-present officer
once released, they should now be grouped together.
In this way you can add 63 more groups considered to be on the Resistance side, hostile to West.
More complicated methods would be to:
group individuals together but use commands such as setPos and commandMove to get them where you want them, but that risks the AI screwing it up.
use scripting to peel individuals off from a distant group of reinforcements and setPos them into position once the number of groups on that side has been reduced.