Selecting the "everyone" group in the allowed_on_groups setting does not enable event creation

[nathank (Nathan Kershaw)]

This has cropped up again:

@david

“everyone” is a little unusual because it’s not a “real” group. It requires plugins to have special handling, which may not have been done here.

I think that this will trip up most people who use this plugin, unless they know that little trick about “everyone”; perhaps simply hiding the “everyone” option from the options would be a quick way to fix this.