I have more than one business. Can I create an Administrator restricted to only one business?
No. An Administrator administers the account or installation. But you can create restricted users who have access to everything in a business.
As @Tut suggested create a restricted user, it allows you to select one or more businesses that user has access to. Once added click on the business name displayed under the restricted user in the users list. Then change permissions: 1) change from full access to limited access and check the items you want the user to “manage”. Full access allows that user all admin privileges for that business.