Letting the people coming invite others

On public and RSVP events, anyone who is confirmed can invite people as well as you. It is on by default, because the people coming to your event know people who would like it, and they are a better source of guests than strangers finding you in search.

What it does not do

It does not let anybody past you. On an Open event their invitee can come straight in — but so could anyone, so nothing was given away. On a Request to Join event their invitee lands in your approval queue exactly like anyone else, marked Invited by @them.

That mark is the useful part. Approving a stranger is a guess; approving somebody your confirmed guest vouched for is not.

Limits

• •Ten invites per person per event. You have no limit on your own event.
• •Only people who are confirmed can invite — somebody still waiting on your approval cannot bring others in ahead of themselves.
• •Everything else still applies: people who asked not to be contacted are never reached, and nobody is invited twice.

Turning it off

Edit the event and switch off People coming can invite others. On a private event it is off already — there you do the inviting, and guests can ask to bring someone instead.