Not that I know of.
One solution: put
disabledAI=1;
in your description.ext file. This will, however, disable
all AI 'playable' units, so it might not be ideal for your mission - it will guarantee you a player leader though.
If you really want a group of a certain size, do this:
Disable the AI with the description.ext
Create a backup group of the same size as your original group of playables less one.
Once the mission starts, count the units in the player's group, then use the 'join' command (unit join group) to fill the player's group out to the number you want. Deletevehicle any remaining backups.
There may be a much more elegant solution, but I don't know what it is...