How to build up a social community membership from scratch?

To build an online community you need to show your expertise and leadership in the subject. You can start by blogging content in the subject and start attracting interested people in the subject. It is a rule that the number of people that reads is 3 to 5 times the number of people that collaborate in the site. You break this rule by having more and more people involved in the discussions and when you have great quality producers of content sharing ideas and answering questions you are on track to grow the community.
From the investment side, it is a very complex issue because this subject the people needs a higher level of trust of the people they are taking. I foresight a great challenge in sharing information about deals, then I will find how the share the learning of the subject as a way to gain trust.

We have a client Rowperfect.co.uk which we've just moved onto a new format for community building.

We created an experts panel (rowing coaches in this case) and are promoting their services, publicizing their blog posts on the Rowperfect platform and providing them with a private Hangout space to learn more from each other.

The mutual benefit is clear
Rowperfect gains by association with famous coaches
Coaches gain by learning from each other and from the audience Rowperfect can deliver.

We have a couple more "ideas" which are better suited for a community whose membership is not yet very large - but I'll hold them back until you get in touch.