Share in sign language looks like you are dividing something up into portions. You take your non-dominant arm with hand open and perpendicular to your body, with thumb pointing up. Your dominant hand will make a loose T-shape with your non-dominant hand, as it sits on top of the other and waves back and forth, as if dividing up a little bunch of morsels.
The concept of sharing can be reinforced with older toddlers by using the sign. Remember, share should not apply just to the toddler sharing their things with others, but also with you sharing with your toddler. By modeling sharing with the toddler, you show them that sharing is something they should want to do, and that they gain by sharing.
This is a really fun sign, useful when we are working on shaping the character of our little ones and teaching generosity as a key value.