is this gramatically correct?

It sounds fine to me, but how about:

For our guests' (don't know where the apostrophe goes) convenience, a block of rooms has been reserved

Hey Amy,

I made up cards for our guests and they said:

For your convenience, rooms have been reserved,
at a discounted rate, at the:


Floral Park Motor Lodge
Address
City, State Zip
phone #

www.floralparkmotorlodge.com


To take advantage of the discount rate,
please specify that you are reserving for:
(my maiden name) / (his last name) Wedding

Directions are on the reverse side

The problem with the grammar is that you refer to the same people in the first and third person(s) within the same sentence.

I think either suggestion that Nina and Cira offered will work fine. Good luck!