they are not going to ripen on the tree. you have to pick them up and keep them for a week for them to be ready. Keeping them in the fridge will slow the ripening process. So if you have too many avocado ready at once you can put them in the fridge for a few days.

I hope you also have a lime tree nearby because you'll need it with an these avocados.

To my knowledge there is not much you can do to preserve them, the only thing to do is to eat then fresh and to give all the extra to your friends and neighbors.

The only way to preserve avocado for a bit longer would be avocado oil but it's a long process to make it at home.