How many people are needed to start the event?

I've seen plenty of people gathered around without the event actually starting. I'm assuming there's a minimum number of people require to participate. Has anyone figured out the number? I've had 12 people gathered for the event and it still wasn't enough, apparently.

16 I think?

My understanding is as soon as 16 people join the group, the match counts down and begins.

After the 16th person joins, a new group starts forming.