* They post all the content themselves under their own name, until other people start joining. It's not elegant and may not look great at first, but it's worked before.
* Fake accounts are used, with the founder(s) pretending to be a number of individuals. Said fake accounts usually get retired as the site gets more popular (for real).
* Previous friends or contacts are involved. Perhaps they run a company and everyone employed is told to post, maybe they're a celebrity with a fanbase, you know the drill.
* Influencer marketing is used. In other words, they try and incentivise celebrities and popular figures to use the site, assuming their fanbase will follow.
* Promotion sites or services are another option, with a few admins doing 'exchanges' for a bit of extra activity. So you go up to someone else running a similar site, and say that if they post on your site, you'll post on theirs for a while.
* They could also just pay people to post. You can hire random freelancers to post content if you're desperate, or use a paid service where people earn money for signing up and posting X amount of content on so many sites.
* If they're really interested in morally dubious tactics, they can also use tools that imports content from third party sites en masse. Seen a few do this with Yahoo Answers, and I wouldn't be surprised if this stuff existed for other services too.
* They could also buy another site (or twenty) and merge them in to get the appearance of a large community. Seen that happen a lot with tech and gaming sites.
* Finally, they could always just directly incentivise people to join by say, offering money to whoever posts X amount of content or refers lots of new users.
Really, it depends on the site in question. Reddit was initially built on fake accounts, but that's only one of about a million ways such sites could take off.