Inventors can lend gadgets to their friends. Wizards can cast helpful benedictions on their allies or summoned minions. Vampires and werewolves might infect normal victims, turning them into creatures of the night.
If the recipient doesn't already have an appropriate character aspect ("friends with Gadgeteer Greg", "Latent witch blood", or whatever), an aspect can be created on the recipient that justifies adding new power facts. If this would deprive the original wielder of the power fact - like an inventor giving away a one-of-a-kind prototype - then this aspect deprives him of the ability to use the power until he gets it back.