Step 1 - Go to individual groups and click Edit Settings. Under 'Upcoming Events' section make sure the Allow HTML Event Details is checked.
Step 2 - Now Hit Submit.
Step 3 - Now go into the Club and Add an Event. The Event Description looks like the Editor and you can add links and images just like how you would do while adding an event in your Syndicated Calendar.
Also choose the below options if you don’t see that option, go to the edit mode of the Club and Group page, and go down to Club Types. Click on each club type and make sure 'Allow HTML Event Details' is checked.

Also to make sure that the user wants to sees the HTML editor when creating the events they have to make sure :
1) if user want to see the HTML event details editor, he/she has to login by one account, we will not show HTML editor to any anonymous user;
2) the user has to join the club first(You could see "Join the club" or "Request one invitation" link at bottom of every club page) before add one event if you want to see HTML editor.