Children are at risk for SIDS until they reach age 1. But that being said some families successfully co-sleep with children under age 1. I co-slept at times with mine until around 6 months but then we were waking each other up. Where is she sleeping now? Are you nursing her?
That being said, I have to respectfully disagree about just letting her cry it out. You can not just leave a baby to cry. She does not know how to sleep. She needs to be taught how to sleep not taught "mom left me here all alone to scream. Why won't she come to rescue me like she always does when I need her?" It's not good for baby and not good for mom. It floods baby's little system with stress hormones.
She needs to learn how to sleep and yes, there are tears when they are learning to sleep but it's nothing like the Ferber method or letting them cry it out. Check out the No Cry Sleep Solution, The Baby Whisperer and Dr. Sears Baby Sleep Book. I was able to gain valuable info from all 3 to come up with a sleep plan that worked for my babies.
You teach a baby how to sleep by developing a routine at nap and bedtime. You literally do and say the same things every single time. Baby then learns what to expect and what is expected. Try feeding, bath, getting ready for bed, soften the lights, soft music and rock baby to get her drowsy for bed. I always held mine till they were into their sleep cycle (about 10 minutes) and then put then slowly down in bed. If they woke up I would stay by them and put my head down on the crib rail so they knew I was still there and they were ok.
You might also try a wedge designed for the crib that fits under the sheet. If baby has a bit of reflux it can make sleeping flat uncomfortable.