Ok, let's get this clear for a minute: The closest we have to a solution so far is dummy heroes that upon selection, select the corresponding Unit Group.
Problem with this method: Max of 5 groups due to a max limit of 5 hero icons.
Solution: Either you just live with 5 groups, or you make it more complicated. For the latter one we stick with the same idea, but we make groups and sub-groups. F1-F5 are the Groups. When one is selected, the icons change (a.k.a. the heroes are replaced) with the ones of the sub-groups. F1-F5 will select a sub-group. This does however requires you to inform the players of this, since new players will not understand that concept. However, this grants every player with 25 Unit Groups. If you
still want more, you should re-think why the hell you should have more then 25 Unit Groups/player. If you find yourself with a good reason, you can make sub-sub-groups, which will allow you with a total of 125 Unit Groups per player.
If you need even more, I have the ultimate solution for your problem: Burn the map on a DVD, destroy the original, throw the DVD in a big bonfire and dance around the flames