You need a lot more than a single meeting. Fortunately, someone’s put the word membrane in the heading for this topic; I hope that someone was you and that your committee has had the roof investigated properly by someone competent.
Any tiles that go on top aren’t crucial to the waterproofing; that’s the membrane’s job. Whatever’s on top of the roof slab now will have to be removed under the direction of someone qualified whom you should by now have appointed to supervise the work.
Then a suitable membrane must be applied to the slab-top as specified by your superintendent. Special attention must be paid to any roof penetration by vents or conduits and the mounting points of airconditioning equipment etc. Then bedding is to be placed on top of the membrane in such a way that the tiles everywhere
will slope towards drains. Your superintendent will inspect the bedding to ensure that water penetrating between the tiles won’t cause cement to leach everywhere like it has in my building. Finally, the tiles are laid on top of the bedding.
You don’t say where your building is. If it’s near the ocean, all the above becomes doubly important because your roof has to endure salt spray.
With 27 units, you may have an owner who’s qualified to review the contract for you. If not, ask your lawyer to eyeball the contracts and warn you what risks you carry. Check that both tenderers have current adequate insurance.
Unfortunately, the NSW authorities have not seen fit to introduce licensing for building consultants so you’ll have to find someone who’s good at waterproofing and legally able to work for you.
Finally, the tiles etc. will become part of the roof. You’re not adding to common property so I don’t think you need a special majority. I’d suggest your committee should have already obtained competent independent advice and should be able to select a contractor.