Note: RSVPMaker 2.5 makes more advanced customization of the RSVP form available from within the administration console, which may eliminate the need for some code-level customization.
RSVPMaker includes a mechanism for allowing you to add custom functions or override the some of the default functions included with the plugin. Specifically, you can override any of the functions defined in rsvpmaker-plugabble.php. You do this by adding your own rsvpmaker-custom.php file to the plugins directory (the directory above the rsvpmaker folder). My distribution includes a sample rsvpmaker-custom.php file but it will not be activated until you move it to the new location.
Essentially, this lets you plug in your own functions within my plugin. The idea is to provide some freedom to customize, without losing the benefit of upgrades to the core rsvpmaker code.
Modifying the event_content function
More extensive customizations are possible by overriding the event_content function, although this is a longer, more complex function. If you open the rsvpmaker-plugabble.php file, you will see the functions are wrapped with if(!function_exists(‘function_name’) ).
If you copy the function definition for event_content into your custom.php file, you will be able to modify the function as necessary for your purposes.