@Erica1323 I think for insurance purposes, your liability issues would about be the same as if a group of 10 booked your home and used your listed amenities (fire pit, kayaks, etc) but didn't refer to their stay as a "retreat."
But where you could run into trouble is if the organizer is running retreats as a commercial enterprise, rather than an activity with a peer group. Also, not to sound too cynical, but be aware that large group guests aren't always honest about their intentions, and requests like these often turn out to be Trojan Horses for parties and excess visitors.