I think part of the reason for that is that most folks consider roleplay to be either spontaneous events or the province of individual expression. They don't tend to consider roleplay as something that's organized as a group or guided by a general plan. However, as many folks with pen-and-paper tabletop games will tell you, roleplay often works best when you have a theme and a group of people who all buy into that theme.
You don't necessarily need to get a brand new guild together in order to form a troupe. It could be something as simple as a small group of five people who want to play out a particular story. Also, a pre-scheduled "bar night" in your faction's capital can be considered part of a troupe, especially if you set up certain ground rules about how things "work" in that tavern.
We've talked before about how to find your own roleplay, but that's not entirely the same thing as starting a new group. Let's jump behind the cut and talk about how to form a roleplay troupe.
Have a purpose or theme in mind
Before you even start recruiting other players, it would be helpful for you to know what kind of theme you're aiming to roleplay. Are you about to tell a love story? Are you going to focus on the tales of war-torn heroes? Heck, you could even be doing a group of characters who have all been drastically effected by the vampire bite of the San'layn.
The idea here isn't that you need to write a long, complex story. You're not trying to be a script-writer recruiting actors. Rather, you're setting up a general context for your fellow roleplayers to live inside. There are definite memes and flavors to different types of roleplay, and it's helpful to have an idea of "where you're going with all this." This information will help players determine how their characters will react to things, how they'll think about things, and what their next steps would be. By way of analogy, characters in an action story just act differently than characters in a romantic comedy.
Part of deciding what your theme is going to be involves deciding whether you're roleplaying a specific story or just a genre. An example of a contemporary story might be that your troupe is an elite team of soldiers going to fight the Lich King. There's a specific action and plot to your story. An example of a genre-based troupe is a "pub night." (Pub nights are incredibly common across roleplay servers.)
Pub nights involve about a dozen characters showing up to a bar or tavern, and just talking and interacting through the night. Usually, someone's playing the bartender or servers, and some folks might even play the chef in the back room. Regardless of how you organize the pub night, the whole thing is mostly based on the idea of a stable, relaxed environment to do some free-form roleplay.
There's as many themes and purposes for potential troupes as there are players. It will ultimately be helpful to you if you can tell new potential roleplayers what kind of story you're playing through.
Decide whether there's a terminus to your story or not
There's two ways to consider the life-time of a roleplay group. Does your story lack any pre-defined end? Or will the story end, allowing everyone to separate and pursue other options for their roleplay fix. I think this comes down to an awful lot of personal preference.
Most "open" roleplay stories lack any cohesive end. After all, you're simply characterizing your toon, interacting with one another, and focusing on the "come what may." Stories come and go according to the whims of spontaneous roleplay, with new ideas coming and going almost every night.
By comparison, "terminating" stories have a beginning, middle, and end. For example, if your roleplay group is about the struggles and challenges involved with killing Arthas, your story is probably (mostly) over after the Lich King dies. Now, it's important to spell out that the end to one story doesn't mean you can't start a new one. It simply means that the previous story has ended and you're stepping into new, uncharted territory.
I'm personally a fan of stories with an end. Like I said, preferences for one style or another is mostly a personal choice. But I like the idea that I can work hard, get very involved, and really bust my hump to make a single troupe story as fantastic as possible. And then, when we've gotten through our climax, it's over. I can relax, take some time off, and recharge before my next story. Open, non-terminating stories don't have that same pattern of release, and often leaves me feeling exhausted and burned out by the time it's all over.
A lot of your recruitment steps are the same for roleplay troupes as they are for raids, guilds, and anything else for which you would want to recruit people. The fundamental ideas are going to be the same.
Check the official forums to see if there are people on your server looking for new roleplay opportunities. While most of the official forums are focused on finding raids, roleplay servers have people searching for other avenues of game.
Take a few minutes to talk about your new roleplay troupe on the Trade Channel. If you can dodge the inevitable Chuck Norris and murloc jokes, you might be able to find someone who is interested in your story ideas.
Lastly, don't be afraid to hit up popular WoW news sites and forums. Especially if your troupe idea doesn't require a level 80 character, you could find yourself pleasantly surprised by how many players are willing to join you on your server of choice.
The last thing I want to reinforce is that when you're forming your new roleplay troupe, you have to do a good job setting expectations. Spend some time to clearly communicate your goals to your fellow players, as well as what kind of operating schedule you might be considering. An ounce of preventation is worth a pound of cure, and making sure everyone knows what they're getting involved with beforehand will save you many headaches down the road.