Well, after all it is not about keeping group selection in the party screen, but about keeping functionalities it provides. Is there a possibilty to do it by adding more filters to the groups then?
Here are my ideas of two new filters which should bring back functionalities that were lost by removing the old system:
#1: Troop type/hero filter.
Description / functionality - This filter would be in a form of a list that contains specific troop types and heroes which are valid type for picked group. On this list a player would be able to allow, or disallow putting into the group certain troop types and heroes by selecting them on the list, and using group's sliders after that. There would be a separate lists like this for every unit control group. By default, all troop types and heroes compatible with the type of the parent group, would be allowed on those lists. Changing overall main unit type of the group, like from "infantry" to "cavalry", would reset the list and would kick out invalid troop types from this group. Disallowing troop types also would kick them out from the group, after accepting filter changes by closing the list. Also, it could be useful to add a button for allowing/disallowing all troop types which are located on the list.
Example / user story - The player starts some easy battle and wants to create a new group with only imperial recruits and some heroes to level them up faster.
The player picks group 8 and creates there a new melee infantry group, then clicks on the "troop type list" located in this group. After that, the list extends itself, and shows every unit type and hero under player's command which counts as melee infantry.
The player has in his/her party Imperial Legionaries, Imperial Recruits, Imperial Cataphracts, two brothers and a sister with only melee weapons equipped, and few companions with bows. All heroes in the party are on foot, no horses equipped for them.
In that scenario, opened "troop type list" for group 8 shows only legionaries, recruits and all three main hero's siblings - two brothers and one sister. All of these listed troops and heroes are marked as "allowed". At this point, moving any sliders from melee infantry groups can put these troops and heroes into group 8. As we know, the player doesn't want Legionaries in this group, so he/she finds legionaries on the list and clicks on them to disallow them in this group. The player also clicks on the main hero's sister in the list, because she doesn't have good armor yet and might easily die in a fight. Legionaries and main hero's sister are now marked as "disallowed" and cannot be put into this group. After that, the player closes the list which makes this filter's changes to be applied to the unit group.
Now, the player adds only wanted units into group 8 by simply moving its slider to the farthest right position. He/she could also have done that by moving to the farthest left slider of group 1, in which all infantry melee units were put at the start of the battle.
#2: Hero party/other parties filter.
Description / functionality - This filter would be useful in battles in which the player can command troops from the other parties, like being the leader of an army for example. It would allow the player to separate his/her troops and heroes from the troops of the other parties. This filter could have three or two states picked separately for each group: "allow only troops from own party", "allow any troops", and optional "allow only troops from the other parties". "Allow any troops" would be the starting, default state.
Example / user story - A player is leading an army and wants to minimize loses in a big battle. The idea he/she has is to take the first, hardest hit in the battle with his/her own infantry units, because high Medicine skill gets them mostly unconscious instead of getting them killed. Other lords' Medicine skill is significantly lower which makes their units more vulnerable.
The player wants to make an infantry group responsible for tanking most of the damage by putting all his/her Imperial Legionaries into group 8. The problem is, the other lords in the army also have Legionaries in their parties. Luckly, there is a filter for this. The player sets it to "Allow only troops from own party" for group 8, and doesn't change it for the other groups leaving their filters on the default state "Allow all troops".
Just like in the earlier example, moving the slider of the group is all the player needs to do after setting the right filter.
I think this would be a better solution than just adding one group for heroes and calling it a day, but maybe I am wrong. I am open to other ideas. I also think that proposed by others addition of one group for heroes still would be better than leaving new Order of Battle system at the current state.