Event timestamp and duration can be obtained in the Rule’s then part:

// declare the accessor interface, it will be automatically injected
global co.streamx.kie.api.runtime.Events events;

rule "GroupCreate"
when
    // for every new EventA
    $a: EventA(!seen, $time: eventTime) from entry-point events
    // check there is no group
    not (exists(Group(eventTime == $time)))
then
    // getTimestamp method accepts event instance and entry point name
    long timestamp = events.getTimestamp($a, "events"));

    // getDuration method accepts event instance and entry point name
    long duration= events.getDuration($a, "events"));
end