Is it safe to share makeup brushes with my friends, without disinfecting them first between uses?

Not really. Unless you have cleaned and sanitized your brushes before lending them to a friend, you might be passing on the microbes that live on your skin. Your body might be totally fine with those microbes, but chances are they will be foreign to your friend and could cause a reaction. Likewise, don't use a friends makeup brushes yourself, unless you have thoroughly cleansed and sanitized them. It won't kill you to use a dirty brush, but it is about the same health danger as touching doorknobs, shaking peoples hands and then licking your dirty hand.