If you want someone to join an online community, try talking about them. People are staggeringly self-interested.
If you want a group of people to join your online community, talk about the individuals within the group.
If I wanted employees of Edelman to join my community, I would start debates about who my members most admired at the organisation. Who’s the most overrated Edelman employee. Who would be the biggest loss to Edelman. Maybe rate Edelman’s prominent bloggers out of 10.
It’s good content (content about people usually is) and it’s likely to be passed around the offices.
The stronger the emotional pull, the easier it is to overcome the joining barriers.