It's just more a matter of how to get people interested in your role play. If you just start the role play without telling people about your idea or sharing a general summary of the plotline (by general I mean a 1-2 sentence tl;dr summary), people will be confused on how to just 'jump in'. The role players want to know that the role play has somewhat of a structure, so they know how to create a character suitable for the role play.
If you're interested in getting more people to join your roleplay, then I hope you will keep reading ^.^
There's 2 ways to grab attention:
1) Usually, and by usually I mean 100% of the time, people post the OOC thread first. The OOC thread would include a rather extensive overview of what the idea is, and how people go about signing up (typically you want people to fill out a character bio sheet).
There's also the option to take things an even further step back and post a brief summary of your idea as an 'Interest' topic in the
Roleplay Topic section, to see if a certain idea for a roleplay is something others are likely to participate in. If you find that indeed people are interested, then an OOC can be made in the
Signups & Plot Discussions topic.
THEN, after a bit of coordination with your fellow roleplayers, you make the IC thread in it's appropriate category.
(It would be helpful if the first post in this thread was edited with a short summary of what exactly your role play is about.)
2) The other alternative is to post it in the
Jump In subforum, where you don't need people to know the entire story, just make it up as they go (well, that's how I see the Jump In forum as XD).