Hi, I've moved this question to the Help, Tips & Tricks section where it seems to fit better.
I've not played D:OS2 enough to remember how it handles group formations, but if you mean what I think you mean, BG3 doesn't let you set up custom formations. You can change who is leading your party by selecting them, and by messing around a bit you can get people in the diamond formation you want, though generally I don't think it's worthwhile as you'll lose your formation next time you change leader.
In general, the game clearly expects you to take individual control of your party members and position them separately when this is important, eg when preparing for combat.
A number of people have said they'd prefer a formation system, and this is one of the topics that's discussed in the Party Movement Mechanic megathread at
https://forums.larian.com/ubbthreads.php?ubb=showflat&Number=679414.
