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This may sound like a bit of a cop out, but I don't think Madam Mvashti would be able to bring druids together in that short of time - as far as we know, she is the only member of her circle in the Sandpoint Hinterlands, and she's likely contacting people as far as Ember Lake and beyond. The party will likely need to deal with Nualia before then, and they have plenty of resources on their side to do so.
That's not to say that this convention shouldn't happen - far from it. Madam Mvashti is a servant of Pharasma, and members of her circle likely share her philosophy re: undead as an affront to the natural order. Assuming the circle convenes sometime early in Book 2, the members of the circle could provide advice and spellcasting services.
Here are some things I think could help get you started:
1) Politics - Who are the highest ranking members of the Circle who are able to attend? What are their philosophies, and what sets them apart from each other, Madam Mvashti, and the PC druid? Who, for instance, thinks this is a waste of time but respects the Madam too much not to attend? Circles may be loosely connected by an ideology or ideologies, but at their cores, druids tend to be independently-minded, at least as far as other humanoids go.
2) Capabilities & Interference - How much can, and how much will, these druids do to help the party? A certain amount they'll do on good faith (such as casting remove disease on infected individuals, or calling on their patron to protect the town). But beyond that, they may need proof that the town (or the hinterlands themselves) are in legitimate danger. For those of the Pharasman faith, evidence of a ghoul plague is probably plenty to mobilize them to containing it and protecting local farms (and the Whisperwood, a fey forest not far from the source of the infestation). Others might need more (or less) convincing.
3) The Ritual - For the ritual itself, I'd recommend using the Occult Ritual rules from Occult Adventures, or at least basing what you're doing on that. Something like:
Your PC druid (and perhaps others, if anyone has come to trust them) are invited to perform in a ritual of protection against undeath (or perhaps evil outsiders, if you really want them in Book 1). They'll perform a skill check or two, and maybe have their magic drained for a day, and the perimeter of Sandpoint (perhaps the water boundary?) will repel the chosen type of creature similar to undeath ward or circle of protection from evil re: summoned evil creatures for a number of weeks. Maybe something as simple as preventing undead to raise within the town's boundaries, so the threat of the farmer ghouls attacking the town remains.
Sorry I don't have any recommendations more specific than that, but I hope that helps with the writer's block!