Compare these two questions below to a recipient of your organization's mailing list.
"Would you like to join a new online community?"
"Would you like to be a founder of our exclusive community?
There are many different appeals you can use.
The most successful, we've found, is using founders (or co-founders).
It uses the efficacy motivation. It sets expectations that contributions and effort will be responsible. And it also hints there will be rewards and recognition.
You can't fake this. You do need to give founders real ownership over the community. This means soliciting their ideas, asking for support on topics, giving them responsibility for areas of the site.
There are two major benefits here. First, you get a core group of members to launch the platform. Second, founders are likely to invite everyone they know to their community.
It's not just a tiny terminological change. It's an entirely different approach. You're building a community with founders, not for members.