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You just need some good preparation and a few opportunities.
First of all, as far as searching for a roommate situation, the best resource is Craigslist, they have a roommate section. Although you'll get a lot of spam there as well. You can both post what you are looking for as well as search for what already exists.
Next, you should have realistic expectations for what you can get for $850/mo. If your goal is to live in Cambridge, then the $850/mo budget isn't enough for a 2bd, but possibly a 3 or 4 bed, but then you would have been be far from the Redline. Maybe East Cambridge or North Cambridge. I know of one two bed in North Cambridge for $1750 and one for $1800 (which could become a 3bd for $2000). You should also be looking in Somerville.
Additionally---how certain are you on living with a large group of strangers? If you can be flexible on your location, it is possible for you to find a place that is driving distance to Cambridge for close the budget you mention. I know there's a studio right now in Medford for $925, I've seen listings in Everett for $800-$995. And the best deal I know of at the moment is a 1 bed condo with HT/HW/Parking and Swimming Pool in East Arlington for $1175.