You probably know more about these sites than I do, but a friend used this particular one to rent an apartment in Paris. Here's an apartment in Zagreb that sleeps 6 people and is $86 a night.
Apartments like this could be an option for people. I like that you get a kitchen and a living room, it might be more comfortable for people staying in Zagreb for a while.
There's also a Best Western in Zagreb. Maybe you could book a block and get wedding rates at a hotel like this (or another hotel).