Identify why freelancers would be willing to join a paid community when there are so many free communities already available to them. In order to get people to join and stay they must know what value you can bring that they can't get elsewhere without paying.

It is difficult to keep people active even in free communities. Charging them for the privilege is a tougher challenge. Either way, it is typically the leader(s) who keep things going. In every group I've ever joined or started when I stopped driving the bus the groups faded and died.

You may be, hire, or partner with strong leaders who become a major attraction. Freelancing is very broad, so unless you bring on other experts you might consider narrowing your focus to a specific type of freelancing skill or at least start out in a particular industry.