Unfortunately, the grid is already assigned.

But rather than increasing the mobs, why not do what I do (i also have at times a very large group).

Look at the XP for the encounter and then divide by 5 (the default number of players), this gives you the XP per player. To make the encouter more balanced for a large group, simply add a monster of that value to the encounter.

Example, if the encounter value is 1500, and it is comprised of two monsters worth 250 points, one valued at 400 points and one valued at 600 points. You however have 7 players. So for 7 players for the encounter to have the same 'worth' you would need an encouter worth (1500/5=300. 300*7=2100) 2100 points. This is 600 points more than the current encounter. You could add 1 600 point monster, or 1 (400) + 1 (250) monster (close to 600) or 3 250 point monsters. You have a lot of choices to the encounter.