Another possibility could be looking into office spaces and startup incubators that rent out small (and crappy) offices for new startups. The rent is super cheap and you usually get fast internet and access to conference rooms. This option depends on where you're located though, as not every city will have these.
I like having somewhere else to work but I don't really like traditional office space. The best spaces I've worked at were a huge loft style apartment and an old house. Thus I'd suggest a shared apartment, BUT:
One thing about office space vs. apartment space is that office space is usually much more negotiable than apartment space. You can probably talk away the $6K advance and lower the price. Aside from the times during boom economies and maybe always on the island of Manhattan, office landlords are often borderline desperate.
On the other side of the coin, if you're looking at living spaces and suggest to the landlord you're planning on using the apartment for an office, they probably won't rent it to you. So don't mention it.
Another thing you can do is sublease a corner of a space where another company got stuck in an overpriced lease last year.
I know my market is cheap, but you can get a 400 sq ft studio here for $400/mo (with everything but electricity and internet included).
At this stage it doesn't need to be 'commercial office space' to function as an office. I'd imagine any room that you'd all fit in would be sufficient.
Alternative: most cities, towns even, have parks of steel buildings with garage space and a small office, pretty cheap. Also executive suites, they are expensive by the square foot, but cheap overall and you get to use conference rooms, etc.