Don’t compete head on with an established community.
You need a different angle. If there is already an established community in your sector you needs yours to be different. It might be more exclusive (Facebook -vs- MySpace), it might be less exclusive. It might be more focused on a particular area, or have a broader reach. It might overlap with another field.
There are many communities for doctors out there. But what about doctors based in a specific country or with a particular speciality? What about retired doctors? What about trainee doctors? What about doctors using new technology? What about expat doctors? What about doctors in high level positions.
You need a focus or an angle that the existing community doesn’t have.
Positioning – your new community effort will fail without it. Pick your position wisely.