I created a new unit Healer, then when to C:\Documents and Settings\Admin\My Documents\My Games\Elemental\Units
Edited the file Healer_1155726780-6502.xml
Added <AllowGrouping>0</AllowGrouping> to the end of UnitType like below.
<UnitType>
.
. ...
<AllowGrouping>0</AllowGrouping>
</UnitType>
</UnitTypes>
Saved file, restarted the game and tried to build the unit Healer, and got more than one!