We had a very successful carol sing here in Storvik back in January, at our Yule Revel. I'm hoping the maypole will also go well. But getting a spontaneous group participation activity can be rough. Ask any dancing master who's trying to round up enough warm bodies to dance Rufty Tufty.
My thoughts, as yet largely untried:
Advertising helps but is insufficient. It's never bad to get the word out, but people forget and lose track of time. Don't depend on a line or two in the site pamphlet to draw people over to your activity.
"Oyez, Oyez" is a start, but... Having heralds announce an activity is good, but isn't going to be enough to convince fence-sitters that they want to try this.
Personal, directed invitations are the best. If I were to do a maypole at an event, I would gather 1 or 2 other cohorts and begin marching or skipping through the site, with a tambourine and perhaps flute, stopping at groups of people to invite them to dance the maypole, come dance the maypole!
Avoid competition, including with the weather. If you are opposite another activity, people can't do both. If you have enough attendance, or distinct groups of participants (e.g., very young children's activities do not compete with Youth Fighter do not compete with adult A&S), this isn't so much a problem. Time blocks before and after court and before and right after feast are also often not good, as people are setting up or breaking down. In the summer, consider that few people over the age of eight are going to feel like running around in high heat and humidity - go early or late. If rain is forcing people under shelters, consider a spontaneous singalong.
Make the barrier to entry as low as possible. Sing well-known songs and pass out lyrics sheets. Promise simple dances and follow through. We once held a bardic circle in a mundane college coffeehouse and got some mundanes to participate, by leaving books of short stories and poetry on a table. They could pick one up and read a selection for the crowd. Offer sanctuary in numbers, if possible - people who might never sing solo will sing as part of a group. Use familiar material if possible.
Don't sweat the details. Relax, have a good time, and don't worry if the song isn't note-perfect or the dance has some bobbles. It's not a stage performance, it's an activity. Encourage people to take a similar view - have fun, screw up, laugh and do it again. The more it feels like a party game and the less it feels like something their 5th grade music teacher is making them do, the better it will be received.