If you mean what I think you mean, you can already do that through the Party menu on the map screen. Unfortunately, though, as it is now, it doesn't save, so every time you load your game you have to reorganize it all. There are 9(?) groupings (corresponding to the 1-9 keys for hotkeying in battle). By default, 1 is all infantry troops, 2 is all foot archers, and 3 is all cavalry (4 might be horse archers, haven't worked with them yet), but you can reassign companions and individual troop units to whatever numbers you want. I find this particularly effective for training large numbers of recruits/tier 2 troops against looters. Just set them to a separate formation number and tell everyone else to stay out of it xD
It can also be useful for, as you said, staging specific groups in battle (i.e. having two infantry groups, one with shields to lead the charge with the more offense-heavy infantry close behind them), companions in their own grouping, different groups of cavalry to flank from different directions, etc.
And if party formations like that aren't what you were talking about... well, ignore me and my dumb self then xD