Edited to add
Even if on the Force.com platform using Salesforce for Outlook, I can't tell you how to access a user's calendar. However, you could still implement a trigger that could email an invitation to users, so they could add the event to their calendars. I believe you'd want to create a template for that purpose so it was clear it was an event invitation.
The code would look something like this:
// This is for a SINGLE email to all recipients in your list
// From your query results, assembled string that holds the email
// addresses to which you are sending the email.
String[] toAddresses = new String[] {'[email protected]'};
String[] ccAddresses = new String[] {'[email protected]'};
// First, reserve email capacity using size() of query results list
Messaging.SingleEmailMessage mail = new Messaging.SingleEmailMessage(queryresults.size());
// Now create a new single email message object
Messaging.SingleEmailMessage mail = new Messaging.SingleEmailMessage();
// Assign the addresses for the To and CC lists to the mail object.
mail.setToAddresses(toAddresses);
mail.setCcAddresses(ccAddresses);
// Specify the address used when the recipients reply to the email.
mail.setReplyTo('[email protected]');
// Specify the name used as the display name.
mail.setSenderDisplayName('Event Organizer');
// Specify the subject line for your email address.
mail.setSubject('New Event Created : '+ Event.Name +' '+ Event.Id);
// Set to True if you want to BCC yourself on the email.
mail.setBccSender(false);
// Optionally append the email signature to the email.
// If true, email address of user executing Apex Code is used.
mail.setUseSignature(false);
// Specify the text content of the email.
mail.setPlainTextBody('You're invited to a new event: ' + Event.Name + ' ' + Event.Id);
// Optionally, you can specify the HTML template you want to use
mail.setTemplateId(value ForId of Event Template);
// Specify the HTML content of the email.
mail.setHtmlBody('You\'re invited to a new event:<b> ' + Event.Name + ' ' + Event.Id + ' </b><p> 'To view the details <a href=https://(your salesesforce na1 instance or URI /'+event.Id+'>click here.</a>');
// Send the email you have created.
Messaging.sendEmail(new Messaging.SingleEmailMessage[] { mail });
It would seem to me that with the right template, the people in your org would be able to use the email as either a meeting invitation which they can calendar or else as a link to an event they can then add to their calendar. I know nothing of how Outlook for Salesforce works to give you any idea of how to link into their API to do anything other than this.