Adding name to guest list for p.i.r.

My son has P.I.R. on 26 JUL 13, he's been writing letters to a young lady (here at home in Texas) and would like her to come I think, but we're only a week away from leaving.  She like to go and has contacted my wife to try to work something out, I'd like to make it happen for him if I can.  I can add a person in the car and the motels with little or no issue, just don't know how to go about getting her name on the list as well.  The form letter states they are allowed 4 guests, he has me, mom and sister listed......it also states the only guests GUARANTEED to see the ceremony are those on the list......it does not state that others won't be allowed.  Not sure how to proceed from here, but letters are our only contact now (as you all well know) and that channel may be a little slow to make this happen.

Any advice?  Thanks in advance.