It depends on your situation with the other person. You can certainly straight out ask, however depending on their spiritual beliefs asking for permission for your energy to access theirs could weird them out. I'd suggest something really simple like " ...do you want to share that with me". Often a hand on their arm is more subconscious but will work. If you don't like the feel of their energy (remember people with issues can have some seriously awful energy), stay outside and use your intuitive skills to help. Don't sacrifice your aura and energy.