I organize the RP LS based on Siren, Tales of Vana'diel. We've been going steady for about two years now, and most of us are planning to be RPing in FFXI for some time. In terms of our theme and whatnot, our RP is sort of a retelling of the game's main storyline, about a group of adventurers getting caught up in all the missions and plotlines and quests, etc. That isn't the entire focus though, its moreso that the game's storyline provides a framework that we build our own storyline off of. We have a lot of 'social RPs' (non-mission centered) for characters to develop themselves and their own plotlines, etc. In terms of our membership, we currently have about 9-12 members who RP on a fairly regular basis.
If you are interested at all, hop on over to our site, where you can check out our forums
(You will need a guildwork account in order to post on our forums, which you can easily sign up for on the site there )
Feel free to sign up, check out the Rules section, ask us questions, and hopefully we'll be in touch!