I am also getting married at the Iberostar Beach in 2013 and was looking at the agreement. You're right, there is a statement about guests at other resorts:
--This package is only for Iberostar Rose Hall Beach guests. The couple and at least 90% of their guests have to be Iberostar guest, (Staying at any of the three
properties in Jamaica) of which at least 80% at the Rosehall Beach
--Any wedding guest that is staying off property will have to pay a day pass fee of $75 US per person non-negotiable.
--Any wedding guest that is staying at our sister property Iberostar Suites or Grand Hotel Rose Hall will be considered as guests of this hotel so only the fee of US$18
A few things to note here - (1) the statement that says that any non-Iberostar guests have a NON-NEGOTIABLE $75 fee. That language isn't used with the statement about the Iberostar guests. (2) I am an event planner at my job, and I have found that almost everything is negotiable. Try to ask the WC if they will waive that fee for your guests. If they won't allow that, try to work a deal out with them. If you have an idea of how much you will be spending at the Iberostar for the entire wedding (say you are paying the $2100 package fee and adding another $2400 on top of that with, additional people fee above 8 people, a cocktail hour, renting out the restaurant for a private reception, etc) and you know you will be spending at least $4500 for the wedding, ask if they will waive that $18/person fee for the suites guest if you agree to spend a minimum of $4000. That way you still get some wiggle room with the costs, and you (or your guests) don't have to pay that extra fee.
It's ridiculous that they even charge that fee for Iberostar guests, especially when the Suites guests have access to the Beach at any other time during their stay. Hopefully you are able to work something out with them.