Most snails don't mind being picked up, especially if they have always been handled. They are very easily picked up and handled but measures should be taken to ensure they don't get injured. It's important never to pick up more than one snail at a time so that one doesn't accidentally fall.
Sometimes people who have never picked up a snail before are afraid it will bite them. First of all, they don't really bite things. They have a tongue like thing called a radula they use to scrape(rasp) away tiny bits of food. Also, they aren't that big. If they decide to rasp at your hand then you'll barely feel it, it may tickle a little but that's all. They only 'taste' you to see if your something tasty to eat, one quick 'nibble' and they decide that your not edible and that's the end of it. Also, some people think snails are slimy and gross and may not want to pick them up by anything but their shell. Please don't! When their just sitting on you hand they just feel slightly cold and damp not slimy! Yes it's true that when they crawl around they leave a slime trail but that's not really slimy either it makes your skin feel funny but that's about it too.
Before you do anything with your snails you'll need to wash your hands to make sure there's nothing on them that may harm them like lotion. Always get your hand damp before picking up your snails because snails dry out very easily and will be reluctant to crawl onto your dry hand.
If you wish to pick up a snail, gently mist it with room temperature water to encourage it to crawl around. Also mist your hand, and then place your hand in front of it so it can crawl onto your hand. If you can avoid it, never pick a snail of any age by the shell! This is especially important for baby, sick and old snails or one with a broken shell.
Never pick up a baby snail by the shell because their shell is extremely fragile. If you need to handle them, it's best to let them crawl onto a leaf of food and then pick this up instead. They also dehydrate quickly so it's better to just watch them crawl around in their house instead.
The lip and where their shell conects into whorls is more delicate than the rest of the shell so always be careful to avoid these areas.
While handling or playing with snails it's important not to flip your hand over so the snail is upside down or in a position that it may fall. Holding it over a soft surface will prevent a major fall accident. If you are walking around with one, you should place it in the palm of you hand with it slightly cupped between both hands. If after a while of holding with your snail your hand gets dry simply mist it and the snail lightly with a misting spray bottle.
It's not advisable to let your snails crawl around and 'graze' outside because of all they things that could happen to them. They could get attacked by a predator of some kind whether it be a bird, other snail or even a cat. They could eat something poisonous or get into something poisonous like chemical fertilizers. They could get dehydrated or any other of a number of things. If you would like to though, you can set their house outside for a few hours in nice weather when it's not to hot or cold. As long as you make sure there in the shade(and stay in the shade!) and protected from predators.
Also, be careful when removing the tank lid if a snail is on it so that they don't accidentally fall. Even though snails need to stay in a slightly damp environment they can drown in the smallest amount of water. They breath air just like us so they should never be submerged in water. Any water they are misted with should be room temperature(not hot/not icy).