Endorsements and calls to join
This is a simple growth tactic for business-orientated communities.
E-mail managing directors, HR representatives, CEOs and other relevant people at large companies. Tell them the community has some terrific knowledge being shared. Would they like to ask their staff to join?
Set up a link where these members can join for free* (and get a bonus/something extra – ideally a summary of the community’s best tips so far).
Once it works at one organization, you can approach their competitors. They don’t want to miss out on letting their employees join for free (or get some unique/exclusive access).
Now do the same for those that run relevant groups, meetups, and others in your sector. Having a trusted person invite their audience to join your community is more effective than you doing it.
* I’m aware most communities are free to join. The act of saying this has a far higher conversion rate.