Cut the group into two. 9 players is just too much. I've played in games like that, and they turn unenjoyable very quickly. When it takes half an hour to play one combat round, people don't have fun.
If you're set on the idea, though, increase both the number and difficulty of their foes. If you just add a bunch of weak opponents, they won't have as much fun-it's just more of the usual. If you up the level of everything by 3, though, they'll run into stuff that will easily take down a few characters. So, for 9 players, add perhaps 2 more opponents, each 1 or 2 levels higher, to challenge them. It helps a lot, though, to try out each encounter yourself. The CR system isn't perfect, and while you can add 5 ECLs of templates and hit dice to one creature without making it much tougher, just a few more levels on a spellcaster can mean the difference between a challenge and TPK.
Also, take a look at how intelligently you're running foes. Use good tactics, like flanking and crowd control, to make what you've got more effective. Don't target the cleric with will-save effects; use them on the fighter. A couple well-placed hold, charm, or dominate effects can also turn the tide against the party.