Sharing makeup is quite dangerous, especially when the one that you share with has infections. Especially for our eyes. The makeup act as the intermediate of bacterium.
However, I understand friends that sharing makeup with each other is normal. But, you need to avoid the special period when some have infection. For other people that you have no idea, it's better not to share.
Eyes are one of the most weak and important place for individuals, so essential protections must be taken. As far as I concerned, it is dangerous to use the same eyeliner. Most people have some basic eye problems even they are unaware of it, such as sensitive to light and hot wind, be afraid of light, eyes are easy to increase tears and feel uncomfortable for something foreign matters. All these may symptoms for some eye diseases, for instance, eye irritation, photophobia. And if your friend who wanna share eyeliner with you suffer conjunctivitis(pink eyes) and red eyes, you'd better refuse her/him with a proper excuse. That's because these types of eye problems are most possible to be infected by Vitus and germs, especially there exists wounds in your eyes. However, it also may be safe. Not all people can get infection when share eyeliner. Just be careful and you should take responsible for your health.