mikestrauss said:
The one caveat I should point out is that NPC camp gets hectic, especially when it is low on NPCs. Often it isn't possible to search for the people who said they would hop when there is a crunch. Or, by the time the crunch hits, it would take too long to gather fence hoppers.
-MS
There is a tool that CT has used recently (last year and change) with amazing sucess that I highly recomend. We created a means of letting all of our PC's know when we would like NPC volunteers that doesn't involve sending people to specifically pull certain PC's. Instead, its a general "Call to Arms" to come NPC if you feel like hopping the fence, where you know that camp has a use for you.
Specifically, we have an NPC walk through the main portion of our site carrying a distinctive (and recognizable) flag. Generally, that NPC wears either the tabard of the local house or some form of armor. The players know OOG that it is an invitation to come down, and IG that the "Supply Lines" need some additional guards, for whatever reason, which creates an IG justification to leave game for a time.
By not pulling (or even approaching) specific PC's, it doesn't push the PC's, interupt any actual encounters, and even gets the word out to more than those people who plot members remember voluneered. It also acts as a convinient plot information tool for rumors, and we've been known to OCCASIONALLY (ie. nowhere near so often as to be the epxectation or norm) use it to hook modules. It also has acted as a good boredom cure, as players that are bored seeing the flag know they can come to NPC camp and find sometihng fun to do (or, just as viable, that something is likely going to happen soon that might entertain them).
All in all, it takes about 5 minutes round trip to "wave the flag" (grab a tabard, the rep, walk through town), adds a minimal amount of environment (the presence of NPC guards being about their business in the adventurer's part of town), and usually brings in 3-12 PC volunteers. We've used it for everything from more bodies for large combat encounters to wanting to have multiple modules and floating encounters that NPC numbers would not support.
The important thing if this tool is going to be used is to bring everyone up to speed beforehand OOG, so that players are aware it is a plot tool, not a module hook or something similar.