Well...it depends.
Close friends (close enough that I will offer to do their makeup for them if they ask, friends that are just acquaintances don't know that I have a fully loaded vanity in my bedroom), my mom, my sisters, I will usually let them *test* things, except they know that they must ASK, use a brush, and not expect me to let them take it out of my vicinity. I don't *loan* makeup. I will do it for you using my own clean brushes that I know have seen soap in the past week, but no, you can't take it anywhere. And don't even ask to use my eyeliner/mascara, okay, that's just gross. I know I'm clean, and I'm pretty sure you're hygienic, but I'd like to keep my germs mine and your germs yours.
There are a few lipgloss exceptions I'm willing to make. However, some of these stories are really making my lips feel all tingly and icky. I can't imagine sharing a lipbalm/stick/gloss with a stranger. I don't know you, so why would you even ask? Some people have no sense of personal space. I remember that in High School I saw a Lancome Juicy Tube on the sidewalk and thought, "Oh that sucks for whoever dropped that," but I didn't throw it away in case she came back looking for it. No matter though, because a girl behind me picked it up and said, "OMG look what I found!" and pocketed it. GROSS is the only word I can think of for that. Simply gross.
If a friend asks to use a lipgloss, I ask them (seriously) if they have the virus for cold sores. I don't, and I don't want to get it anytime soon, thankyouverymuch. If you lie to me and give me one, I will kill you and make it look like an accident. It's a very rare thing that I will share though, most of my friends know that I am kind of OCD about my personal bubble.
So...don't be offended if I don't want to share with you.