A mix of pre-existing relationships and relationships that can naturally form.
And game-wide plots lead by the administration.
Perhaps the easiest for this would be a (fake) TV show- the pre-existing relationships could be the show-based relationships but the organic ones could develop over time.
Have a standard RP post length and stick to it- 300, 500, 700 words or so per reply and ask for people applying to give a made up sample that shows they can do it.