To ensure people can sign on to your event please make sure the event time is set correctly. If the web site thinks the event has already started because the time was set in the past it will not allow people to sign on.
The following steps should help you set the correct time for your event:
1) Specify your own time zone in your profile (see your current time zone below the calendar)
2) Enter time of the event when it happens in your time zone
The web site should automatically convert the times for people in different time zones, as long as they also specify their own time zone in their profile.
Please include the GMT time in the title of your event for better clarity.