Choose Give complimentary access on the Supporters list, enter the person's email address or handle, choose the tier, and optionally an end date and a note only admins see. There is no limit on how many people can have it. They get the same welcome email as any supporter, and the access opens every post their tier opens.
The access ends on its end date, or when End complimentary access is chosen on the supporter's page. Giving the same person a second grant changes their tier and end date.