If the bride and groom have sent you an invite it should have the admittance of the people invited with their names attached. If the person has not received an invitation, and their name is not on the guest list, they are unfortunately not invited.
If people are not on the list, and were not invited personally by the bride and groom, they will not be allowed into the venue. We are filled to capacity, and the people that were invited are intentionally meant to be there.
We'd really love to celebrate with you, but we do need RSVP's by the specified date. If we do not hear from you by then, we'll be offering those spots to other loved ones who are awaiting an invite.