Google Calendar is one of the most useful Google services ever created. A calendar that syncs across all devices is something many people simply can’t live without, especially if for those with a small business. However, it seems that there is a bug affecting Google Calendar right now, so you’ll have to be careful until it’s fixed.
If you create an event from a computer and insert an email address into the subject, Calendar will automatically add that person to the event, even if it’s private. This may be a sort of “smart” feature that adds those who you add via an email address, but it’s a bug when Calendar doesn’t ask you to confirm doing so.
If you use Google Calendar, keep this in mind when creating new events. Put someone’s name instead of their email address in the title, just to be safe (though I don’t know how many people tend to put emails in the subject of calendar events). Hopefully Google will get right on fixing this.