How can I make some guests of my event able to change the event? I would expect an answer in G+ Events help, but there's nothing about this topic.
More admins in Google+ Event
google plus;google plus events
Google+ does not have the ability to allow guests to edit an Event as of Feb 2014. One alternative is to Create a Google+ PageInvite Event managers/administrators to become Manager of your PageAll Google+ Page managers will automatically have the permissions to edit an Event