Go to the “Business details” page (for example, by clicking on the business requested on the “My businesses” list).
In the business menu (on desktops in the left column, on mobiles/tablets in the bar in the lower margin of the screen), select the “Business” item.
In the upper part, click on the “Users” folder.
Insert in the search field the email address you want to add.
In the lower part, you will find the user you are looking for. Then click on “Add”.