The "community and organization" part means that you need to have an existing audience for your site proposal. (If you don't have one, try to find one or join one.) Then, you would have an existing community to link to.
Reddit is probably the best place to look for an eager community, since it not only has experts in the area, it also has an active community of users who can potentially be interested in your post. Quoting @user in my Q&A session in the PLDI.SE chatroom:
The experts just saw the proposal and came, I think
We got like 40 from the reddit community, but the majority was 1 or 2 per community yes
Other methods (like Discord, Facebook, Twitter, ...) might not be as rewarding as reddit, since you don't usually get as many engaged users in these other sites as reddit.
Besides, if you found existing people, it would be even easier to expand your community, and find other engaged communities to build your site. Quoting @mousetail in the same session above:
Some of us where part of these groups and they all link to eachother
If you're organizing a community, it would be helpful to join a lot of different online community sites, so that you can find one or two people who are interested in your topic.
It can also be helpful to work with your friends to build your initial community, and make it known to others.
Do I have to pay someone?
Well, no if you manage to find an existing online community in your subject. You probably don't need to pay to invite the experts because they're interested at sharing information anyway.
Why can't I just create the proposal and then see who is interested?
If your topic doesn't have a commuity, it's not very likely that random people online would gather together to form a successful proposal in a short time, given the time limit. If it's a good topic, it probably has an existing community already.
The prerequisite is to have a community or find an existing one, so that more people would follow or commit your proposal, and use it. If you submit a proposal and then see whether people are interested, it would make the site less likely to survive.