For most of us, LinkedIn is where we connect with our professional contacts.
If you're building a community for a profession, LinkedIn is a good place to start.
Export your LinkedIn contacts (LinkedIn > Connections > Keep in touch > settings > Export LinkedIn Connections) into a CSV folder. You can get your colleagues to do this as well.
Remove those that clearly aren't relevant to your community effort.
Now use this to grow your community. Ask for advice/exertise. Invite them to participate in a discussion. Ask who wants to be founding members etc…
It's far easier to invite those that personally know you to join the community than strangers.