Do you wish to be in the countryside or in the city? All single rooms, some double rooms or even shared apartments? This last one because there are some estates that have several villas, all divided up into apartments, but that could host your large group and maybe you would have the estate to yourself. That is, if some guests would be willing to share apartments, generally the rooms within still have their own bathrooms.
You also don't say WHEN/what period of the year this conference is set for, so not sure whether you want a place in summer and would welcome pools or being near the coast, or closer to the city in the fall. Also, do you have a budget (or not) and therefore are looking for places that are not too high end/luxurious?
Ok, so here are a few general options that cover some of the types you might find that can host minimum 90 people that way you can take a look and evaluate what you might prefer and get a better idea of what you'd like.
At this point I hope these are a start for your search. As you take a look at all of them, develop a better idea of what you're looking for, you can let me know and I can offer more recommendations that meet your criteria.