It has some pretty specific requirements. I doubt it's a mod conflict. At the very least it should show up if:

• You are landed or landless admin.
  
• Female adultery is accepted in your faith.
  
• County rank or higher.
  
• You're a woman attracted to other women.

If you meet those requirements, it should at least appear in the activity list and show you any other blockers. Female adultery must be accepted due to the public nature of the event.

Are there any guest requirements? I can host an event, but it always says that no guests can attend.

They need to be willing to cheat on their partners, for one. One way to guarantee this is with polyamory. Aside from that, they need to be healthy women 18 years of age or older, who are bisexual/lesbian with no unattractive traits. If you have a lot of lesbian lovers you should be able to find a couple of them, but if you don't have polyamory for your religion it's somewhat trickier.