With a background in CS you have the functional hacker skills needed to create a first version of something, but you say your energy lies in marketing skills, so you know what 'something' to build.
I see this as one of the most powerful combinations seen in founders: the skills to get things done, and the soul to see the big picture of what problems people need solving.
On the flip side, the problem you have joining in with another startup is that the founders are often great generalists who need specialist help. Therefore a business/systems analyst is not an obvious fit with a startup, where the hacker-founders shoot first and measure later... at least until they've proven there is something there and they can mature their processes and goals.