I personally built our beta community when launching Vidyard (YC - Summer 2011).
Our model was built around signups on the Vidyard landing page that we'd nurture via e-mail campaigns and eventually add to the product (when we were ready) to stage growth.
I followed and engaged with everyone on Twitter that breathed a word about B2B Video and Video based marketing campaigns. My bio was designed to draw people to our landing page, where a portion of them would sign up for the "beta list".
The metrics worked out (ish) as follows:
100 follows a day --> 20 conversations on twitter --> 1 signup to our "beta list".
I then personally engaged with each signup to validate their need against the product we wanted to build. When ready to launch, we had ~1500 people on the list ready to try the product.
I spent ~6 hours a day focused on this.
Happy to jump on a call to discuss further!