Example of how to add a capability to a user when using the WP User add-on. This function takes all of event's category slugs, loops over them and adds the capability 'access_s2member_ccap_{category_slug}' to the current user.

example.php

<?php //Please do not include the opening PHP tag if you already have one 

function tw_add_custom_ccaps_on_registrations( $user, $attendee, $registration) {
    
    //Check we have instances of the objects we need/expect.
    if (! $user instanceof WP_User) {
        continue;
    }
    if (! $attendee instanceof EE_Attendee) {
        continue;
    }
    if (! $registration instanceof EE_Registration) {
        continue;
    }
    $event = $registration->event();
    if (! $event instanceof EE_Event ) {
        continue;
    }

    //Pull the categories from the event
    $categories = $event->get_all_event_categories();

    //Loop over each category and add a ccap using the category slug
    foreach($categories as $category) {
        if ( $category instanceof EE_Term ) {
            $user->add_cap( 'access_s2member_ccap_' . $category->slug() );
        }
    }
}
add_action('AHEE__EED_WP_Users_SPCO__process_wpuser_for_attendee__user_user_updated', 'tw_add_custom_ccaps_on_registrations', 10, 3);