SECURITY!
Keeping uninvited guests from crashing a wedding is most often the responsibility of the maitre d' or your wedding planner (and team). If you are concerned that there will be a lot of people showing up that weren't accounted for, then you may want to consider designating someone for the sole purpose of guarding the door.
Don't just hire some big scary-looking dude in a tight t-shirt. If you MUST hire security, then hire a reputable and professional company. Hire a team that will match the level of class your wedding deserves. They should blend in with the rest of the party and stay near the doors. (You don't really want an armed guard in uniform, do you?) They should be courteous and polite but also firm and assertive.They should be able to handle any situation discreetly without drawing attention away from the bride and groom.
Hopefully, this is an anomaly and not a sign of the times. To quote Rodney King, "Can't we all just get along?"
A couple of private event security firms in the Philadelphia area: